The journal Science released an article entitled “Decline of the North American avifauna” by Rosenberg et al today (Sep 19, 2019), and already disaster laden headlines are appearing in major newspapers (I’m not going to bother to link to them because they’ll probably change by tomorrow but I bet you’ve already seen this in your favorite news source).
Has North America truly lost almost 3,000,000,000 birds over the last 48 years? I don’t know. But I do know that number which is likely to appear in the most headlines is an estimate and one of the less solid numbers and I will argue one of the least important numbers that appears in this publication. I haven’t had time to fully digest the methods yet myself, but these are credible people and on a skim most of the methods look reasonable (fitting a hiearchical Bayesian model is something I am in the middle of writing a paper doing that myself so I’m hardly going to critique it). And the authors put error bars on everything which is too rare and therefore to be commended. A very quick summary of their methods is they make two estimates and combine them. One estimate is of the trend for each species from an index of relative abundance, often from the high quality breeding bird surveys. And second, they estimate the total number of individuals of each species found in North America (excluding Mexico) from an extrapolation from data covering a fraction of a percent of the US. I don’t have a better method but this is clearly a step that introduces a lot of uncertainty. Then they combine the reliable trends in relative abundance and the less accurate total continental abundances and sum it up across 529 species. That’s how they get 3,000,000,000 bird decrease. So these kinds of estimates require a ton of assumptions and extrapolation which makes it hard to do, but that uncertainty and challenge shouldn’t prevent us from doing this. Even if they are very approximate estimates, we need these kinds of numbers to enter our conversation. And I am not brave enough to provide my own alternative estimate! So these are the best out there.
For all the bird apocalypse headlines we will see, what is really in the data and what does it really say? I’ve downloaded their first spreadsheet from supplemental material (I am basically skipping over the NEXRAD radar data because I have less expertise on that kind of data but they got very parallel messages). So I take as a given their estimate of population size “Estimated North American population during range of years between first_year_popest and last_year_popest” which I will call “AvgPop” (even though it is estimated by a more sophisticated method than average and indeed they have to estimate the population sizes over different years so they couldn’t even average over raw data). I am also taking their “Estimated change in number of breeding individuals over the trend period (usually 1970-2017), based on a combination of current population estimates and long-term trajectories. Median of the posterior distribution from the hierarchical Bayesian model” which I call “Abundance Loss”.
From there you can calculate whether a species increased or decreased (Abundance Loss greater or less than zero) as in this Figure
Percent of species showing increases or decreases
We can also look at the percent increase or decrease of populations given by (Abundance Loss)/(Avg Abundance) (although by the vagaries of the methods used this can show declines of more than 100% which initially seems problematic). This is not really a problem since a true percent loss would have (Abundance Loss)/(Initial Abundance) but initial abundances were not estimated (or at least not reported) in this model. Declines of more than 100% are not incorrect nor a sign of errors, but they are a bit awkward and might give a flavor of how approximate some of the numbers are. If we plot a histogram of percent loss we get:
Histogram of species % change (right graph shows only species with between -150% to +150% change)
These two figures suggest that on a species level, many have increased, many have decreased, most are not too terribly far from no change and the data is not terribly far from centered on no change but in fact the average percent loss is -30.9% (this is not that different than their reported 29% loss but I got there by less sophisticated methods). 31% decline over 48 years sounds pretty big but it is over almost 5 decades so less than 1% (0.64%) loss per year on average. We’re on average losing for sure, but not racing towards zero at break neck speed (it would take another 56 years to further decline from 69% to 50% of original numbers by one overly simplistic calculation).
But here is my first question. Is it even useful to say 30.6% (or more accurately 29%) decline AVERAGED across all species? As the 2nd figure shows that average is almost misleading given how much variation it hides. Wouldn’t we be much better off to say: a few species have declined drastically, a few have increased drastically many haven’t changed that much? This is a general pattern, not just in these birds. And then from a conservation view wouldn’t we be better off homing in on those birds showing out of control declines or increases? Probably yes. But its certainly not a loss to know an overall average is a decline, as long as we don’t get too hung up on boiling down an enormous seething pot of complexity and winners and losers to one number.
Here is where things start to get a bit more complicated. The number that I’m sure will be in most headlines is not -29% but -3,000,000,000, the number of individual birds lost. For starters as already discussed that number comes from combining the trends on indices of relative abundance (which I would consider much more robust data) with estimates of total numbers of individuals in each species across North America (which I would consider a much more difficult and therefore approximate number). So right there the 29% number is probably a more accurately estimated number than the 3,000,000,000. But there is something more going on than one has smaller error bars.
The 3,000,000,000 number is a weighted average on abundance. And there is ENORMOUS variation in abundance. The most abundant bird (by their data) is the American Robin with an estimated 366,076,928 individuals living in North America while the rarest bird is the Dovekie (an auk related to puffins) that has 1,500 individuals (the rarest landbird in the datset which might be a better comparison to the Robin is the Snowy Owl with 14,000 individuals, although the Kirtland’s Warbler is often estimated to have around 2,000 individuals and the California Condor has about 500 individuals, but both species were too rare to make their data set – and both have increased in recent years). The Dovekie has a loss of 55% while the Robin has a gain of 5.5%. Up until now every species counted equally and the Dovekie drove down the average trend more than the Robin drove it up (average of the two gives -25.25%). But now the Dovekie trend gets a weight of 1,500 and the Robin gets a weight of 366 million, completely swamping the Dovekie data. This is the right way to get the total number of individuals lost, but it clearly hints that its going to pretty broadly transform the nature of the results and suddenly it really matters which species are increasing and decreasing as only the really common species are going to matter much in that 3,000,000,000 number. If you’re going to weight your data (even in an objective and purpose driven fashion), you want to understand your weights!
Or put another way, when there are winners and losers, it is natural to ask WHO is winning and losing. Every summary analysis to this point in this blog post has treated a rare species and a common species as of equal weight in the averages (except their 3,000,000,000). It might seem intuitive that the rare species are declining the most and the common species are holding steady or maybe are the increases (and maybe even the increases are invasive species). Indeed the two extreme data points I picked out in the last paragraph seem to confirm this scenario. This would be a worst-case scenario. So lets check (again using their supplemental data). A plot of percent loss versus abundance (with abundance on a log scale) shows:
% loss versus population size – common species decline more, rare species decline less
Again there is a lot of noise, but that black line is sloping weakly down (statistically significantly so if “% loss” is log transformed as it probably should be) – common species are on average declining more than rare species! That paints a very different picture! It still doesn’t change the decline of 29% averaged across species, but the big losers in numbers of individuals are really common species. Mathematically that almost has to be true.
The story gets even more interesting if we dive into some of the biggest losers. If we take the 10 species that have lost the most individuals we get:
Loss of 10 species with biggest decline
We see a couple of things. The birds that are losing the most in absolute numbers (# Lost) are also showing very big % Loss (including several with >100% decline), but they are also among the most abundant birds (Avg Pop). Indeed these birds all rank in the top 39 out of over 500 bird species for how abundant they are, and 4 of the top 10 losers are in the top 10 most abundant. Or as the authors of the Science paper note, “While not optimized for species-level analysis, our model indicates 19 widespread and abundant landbirds (including 2 introduced species) each experienced population reductions of >50 million birds”. Yes indeed! That is in the paper but I guarantee you it won’t make it into any headlines and probably will be mentioned in a very small fraction of all the coverage. Two of the 10 biggest losers are actually invasive species (House Sparrow, European Starling). Indeed those two species are widely regarded as among the worst vertebrate pests introduced to continental North America. In case you are feeling bad for the house sparrow, don’t. It is still found on 6 continents (invasive on four of them) and is the most widespread and second most abundant bird in the world. When an invasive species declines is that a good thing or a bad thing? All I will say is that land managers spend a lot of money trying to achieve this outcome usually without much luck.
Eight of these 10 biggest losers are considered “Least Concern” on the IUCN list (farthest from extinction) while the other two (Common Grackle, Blackpoll Warbler) are Non-Threatened (next farthest category from extinction). Although some of these birds may be threatened in individual states or for particular subspecies, yet at a continental scale the conservation concern is almost nil. Several of these birds are not just widespread and abundant in North America but also abundant on other continents. As noted two of these birds are out of control invasives that may just finally be receiving a long overdue adjustment. The Pine Siskin is an irruptive species (huge population outbursts and crashes) which makes it hard to attribute any long term trend. The Blackpoll Warbler, White-throated Sparrow and Dark-eyed Junco breed in primarily boreal or mixed forests and often in edge, early successional or burned habitat whose frequencies may be changing due to human actions, but they are so common we have really done very little research to understand why they are declining.
The remaining four (Savannah Sparrow, Horned Lark, Common Grackle, Red-winged Blackbird plus the already mentioned invasive House Sparrow and European Starling) all love low intensity farmlands and saw large range and abundance expansions as Europeans cleared>90% of the Eastern and Central US for farms. These 6 birds are now likely declining because of agricultural intensification, but it’s important to note that their decline is from an artificially human-induced higher than pre-European level. They may well all still be above pre-European population levels even after huge decline. Is that bad that such species are declining because of agricultural intensification? Probably? But for sure its not the saddest story. The saddest story is the decline of prairie and forest specialist birds (and in a few cases extinctions) 150-200 years ago when European settlers were clearing that >90% of the land. But that story completely predates this (and nearly all other) data sets and is not often told quantitatively. These specialists were the big losers; the birds at the start of this paragraph were the big winners back then 150 years ago but now are in the top 10 losers. So its a case of the tables turning. Unfortunately, though, its not the case that intensification is returning the land to those original losers. Intense agriculture is a biodiversity desert (although it does feed people). But on the other hand, intensification of agriculture can also concentrate agriculture and lead to less land needed for agriculture which at least has the potential for returning land to more natural habitats (and the proportion of land devoted to agriculture is indeed decreasing in countries like the US and UK although some of this is also due to conversion to urban land). In some regions of the US experiencing reforestation, forest birds are gaining while the farmland birds are losing. So while I wouldn’t call it good news, this is hardly the most disastrous story out there. And these species may still be more abundant than they were before Europeans arrived even after large declines (and certainly are in the case of the two invasives). Its really a rather complex story.
So I may or may not have convinced you that declines in at least six of these ten birds are really a return to a more natural order (and three more of them are so abundant we haven’t yet bothered to study very hard why they’re declining but it might also be changes in already human modified lands and the last might not really be declining in a meaningful sense). But I do want you to notice that total on the “# Lost” column. IE 1.6 billion (>half) of the 3 billion decline is found in the 10 biggest losers that are all extremely abundant widespread, and mostly anthrophillic (human habitat loving). In the same vein with a slightly different approach, look at the 40 most abundant species. Of these 40 maximally abundant species, 31 show declines, and the 40 most abundant species together have a combined net decline of over 2,184,000,000 (or over 2/3 of the total decline are in the 40 most abundant species). Bad? Sure. A sign of human impact on birds being large? Sure. But a conservation disaster worthy of words like armageddon? Probably not unless you are selling newspapers.
So is the average bird species declining (in NA)? Probably yes, and probably by about 30% on average, BUT that hides a lot of variation and treats every bird equally. Is that 3,000,000,000 birds lost correct? That is a big extrapolation from the raw data, but it is the best available guess and I don’t have a better one. But are we in a death spiral where common species and invasives are taking over and rare birds are taking it on the chin? Not in any general or on average sense, no. Quite the opposite, more than half the birds lost are from the 10 biggest losers that are all completely safe, often so widespread they are considered a nuisance, two of which are invasive, and most of which are declining because they got so big by exploiting habitat created/modified by humans in the first place which is now changing on them. Or alternatively the 40 most common species (out of 529) make up over 2/3 of the total 3,000,000,000 decline. So that 3,000,000,000 decline probably is being interpreted by people pretty differently than what is really happening on the ground.
So are humans impacting birds in a negative fashion overall or on average. Pretty evidently yes. The Anthropocene is real. Its hard to take away 50% of the total land and 50% of the freshwater each year and 50% of the new plant growth each year globally and not see declines! Given those statistics its actually kind of surprising the declines are not more. And in a small good news embedded in larger bad news fashion, most of those 3 billion in declines are in species that are not at even the slightest risk of extinction.
So do the statistics in this study really directly address questions of conservation or biodiversity preservation? Not so much (at least not in the headlines). For that you need to go look at birds that are rare and declining. Of course that is what conservationists have been doing all along. And there you get a mixed story. Bald Eagles and Wild Turkeys have made pretty spectacular come backs. And the Kirtland Warbler and California Condor are both naturally rare but have actually increased in recent years due to intensive management efforts so fairly positive results but certainly not victory. But we have made several bird species go extinct including the Carolina Parakeet, Heath Hen, Passenger Pigeon, Labrador Duck and probably the Ivory Billed Woodpecker and a few more (many more if we include Hawaii). And a number of other species are on the ropes (a few dozen endangered bird species in North America, again a disproportionate fraction of them in Hawaii). That is a really important story (both the positive and the negative), but it is an almost wholly different story than -3,000,000,000 or -29%.
So should we be worried by the two new headline results? If so, how worried? That’s a subjective question depending on one’s values around biodiversity, balance between human and biodiversity needs, and conservation goals and priorities and how we weight different aspects. Hopefully I’ve given you some facts to help you come to your own answer. I have my answer (which I have not expressed) and I expect you each have your own slightly different answer. And across all the readers I bet there is a pretty wide range.
So what do you think? What are the headlines you are seeing? What is your interpretation? What do you think is the take home message?
Thanks for writing this, great post. I was asked to comment on this study for media & declined, as I don’t have appropriate bird ecology/US knowledge, but your post has confirmed my suspicions! I also wondered how appropriate the radar data is for assessing changes in community composition/diversity.
Another subjective question, should we also be worried about the potential for feedback loops between increasing coverage of popular apocalypse narratives & the studies that get published & promoted within that framing?
Thanks! I was in no small part inspired to do this by your excellent blogs on Insect Armageddon.
I need to dig into the radar data more today. But I think the highlights are 13% decline in 12 years. But as you note this is a biomass estimate, not individuals. And as far as composition all they can really say are “migratory birds”. So in short, same story in reporting non-trivial declines in total amount of birds. But unable to go into the more nuanced winners/losers/compositional changes that I did for the (primarily) BBS data.
“should we also be worried about the potential for feedback loops between increasing coverage of popular apocalypse narratives & the studies that get published & promoted within that framing?”
Oh there is an important and pointed question. And I would have to say yes. In fact this paper is almost proof of that. All of this data has been kicking around and been analyzed already for years. There are already good BBS papers that show decline in abundance primarily driven by farmland birds with other winners, but constancy in richness. And there have already been radar decline papers too. Its hard not to imagine that this all got repackaged to parallel the insect headlines.
Here’s the 2nd order question that will never get asked. Is it likely that both birds (as a primary consumer of insects) and insects would both massively decline at the same time? Its probably a hard question to ask. Most of the biggest decliners were primarily seed eaters but definitely throw insects into the diet especially when breeding. And I did see one reference to House Sparrows probably being primarily limited by insects when they need protein for their young. But Robins (one of the biggest gainers) are also insect eaters. So probably a simple “box” trophic cascade model (which would say that when birds decline insects should go up) is probably too simplistic. But it is an intriguing question.
Yes I’ve wondered about the parallel bird/insect decline issue. Insect pops can respond much more quickly to resource pulses & environmental changes than birds, so it would be interesting to test this….esp taking into consideration community level factors eg specialist/generalist composition & adaptation to new resources
Robins eat a lot of earthworms and other terrestrial invertebrates and fruit, which most insectivores, most of the time, do not. In analyzing population trends, a more detailed examination of diets might be useful.
All good points.
Nearly all land birds feed insects to their young—most baby birds can’t digest seeds. So insect collapse would specifically affect birds’ replacement rate and long-term population trends.
That’s basically true. Which makes it interesting that some birds are increasing by massive numbers and some other (more) birds are decreasing by massive numbers.
I think the analysis of BBS data was state of the art (i.e., species groups as factors, with species as random effects) and will set a new bar for how future analysts assess population trajectories across large assemblages of species. And future analyses could extend that hierarchy by another level and consider regional variation nested within species, and begin to identify covariates that might be responsible for increases and declines in different species and in different regions. You are correct that the weakest number in the paper is probably -3 billion, but an analysis of myriad factors affecting trajectories of 529 species of North American birds was never going to fly in Science. But evidence of those patterns is buried in panels C and D of Fig. 1, and I look forward to further analyses (and formal critique of this analysis, facilitated by ready access to the data). I also look forward to critiques of the population estimates (the weakest link in the study) and hope it will motivate other approaches: e.g. genetic, capture-mark-recapture.
I agree this paper was clearly an advance for science. It set a high bar in so many ways including its efforts with putting error bars on estimates and sharing the data. And I’m looking forward to digging into the details of the hierchical model.
I hope it was clear in my post that I respect what was done by the authors. My main thoughts are what the journalists are going to do with this (not the responsibility of the scientists). We are going to create an impression that birds are on their way out. And 20 years from now when they’re still very much around but differently composed, that is going to bite our credibility. I still remember when as a child there were headlines about how the 1970s were the start of another ice age. And even though I now teach the science of climate change and have researched to see that careful climate scientists were actually talking about global warming even though it wasn’t getting headlines in the 1970s, it still gives me cognitive dissonance in trusting climate science (priority effects and all that)
So this blog post is just an attempt to add nuance to what I am sure will be sensationalistic coverage.
Thanks for digging into the detail behind this number and for the subsequent debate. I’ve not read the paper (my upgraded OS won’t give me access to Science, for some reason), so I won’t comment on the rights and wrongs of the calculations. I’m really fascinated by how these numbers get interpreted by journalists and others. Does it help or hinder the communication of complex statistics when the results are boiled down to a single number, particularly if that number is in a different currency from what was actually modelled?
I should at this point declare an interest in this topic. I did something very similar back in March, when we published a paper on pollinating insects in Great Britain . Our statistical models showed that bees and hoverflies had lost 25% of their distributional extent over 3 decades. My co-authors agreed that 25% loss is a big number, so we felt comfortable with a title of “widespread declines”. However, I was concerned that non-specialists (and particularly members of the public) would find it hard to grasp what 25% loss actually looks like. So I decided to convert the 25% into a figure for net loss in terms of the number of grid cells. Like the bird calculations that Brian describes, it required a few tenuous assumptions. The answer is 2.7 million 1km grid cells across all 350 species. This is not quite in the same ballpark as 3 billion, but considering Britain is just a quarter of a million sqkm, it works out equivalent to a net loss of 11 species per grid cell.
In both cases, these numbers are supplementary to, and extrapolations of, the main results. In this context, I think they can be very helpful to the communication process, so long as they are appropriately caveated. Within these caveats should be a statement that the study was not designed to estimated change in the actual number of individual birds or bee populations: rather, it’s an approximation. However, I fully take on board Brian’s warning about the potential for misinterpretation.
In fact, I also have experience of being misunderstood, in a wilful and damaging way. For an earlier bee paper , I conducted a supplementary analysis to translate modelled effect sizes into long term ecological change. The resulting numbers formed the basis of large parts of the subsequent press coverage, so I wrote a blog post  to explain their derivation. I was pretty open about the fact that my numbers relied on some assumptions and were not part of the actual analysis. But, given the sensitive subject matter (pesticides), I probably should have anticipated the risk of my words being taken out of context . I did not appreciate having my scientific integrity trolled.
I’d be really interested to hear the views of others on this topic. What should we, as scientists, do to convert the results of our models into currencies and language that are accessible to a lay audience? How can we present these calculations transparently without making them appear simplistic?
It is really hard to know what is the most effective way to engage. Yes social scientists have some basics about going out to stake holders in person, finding connection points etc. But on the questions scientists really care about – what will have moved the needle most on policy and behavioral change 10 years from now, I don’t think we really know. I personally am worried about scientists being too alarmist or exaggerated because the media only amps that up more and while it gets headlines and attention, my fear is longterm it depresses, demotivates people and decreases the credibility of scientists. Scare tactics & doom & gloom didn’t get very far on changing smoking behavior. Or then again maybe it did but it took 2 generations. We’re all just guessing. And I respect anybody who is out there trying.
If I were writing a public consumption summary of this I would say something like 57% of bird species are declining and have declined by 6.6 billion individuals while 43% of bird species are increasing and have increased by 3.6 billion individuals. Most of the action in numbers for both increasing and decreasing individuals is in really common species that are not endangered and very often became common because they get along well with humans.
75 words. Is there any chance that complex a message gets through to the public? Probably not.But maybe. Certainly even if it doesn’t get in a headline that is not too much to communicate in a news paper article. Really how hard would it be to explain that that the net -3billion is actuallya combination of +3.6 billion and loss of -6.6 billion. And yet that difference is so much more nuanced and accurate and inviting of more questions. And its not hard to animate the story with specific species examples (as I did). And surely a policy maker who is getting 10 powerpoint slides can get that level of complexity.
A related question: biodiversity impact studies almost always have a quantity (richness, number of individuals, biomass) and a composition change component. We seem to always only focus on the quantity change and not the composition. Sometimes both are going on (as in this study I would say both are about equally important). but other times the story really is in composition, not in quantity (e.g. the Dornelas 2014 paper on local species richness but also 10% species turnover/decade that I was part of). There was basically no quantity story but to my mind a huge composition story, but people still obsessed about the nonexistent quantity story. Scientists have got to figure out how to get across and get people to care about the composition aspect. There is a lot of important stuff going on there. Would we be happy if there was no net change but it was all Robins and pigeons and crows? I think not. So why can’t we communicate that?
They say a picture paints a thousand words, so maybe part of the answer is to invest a *lot* more effort into data visualisation, graphical abstracts and animated figures for widespread consumption. Few of us have much expertise in this are, and I suspect many of us give little thought beyond the figures that appear in the paper.
I think you’re right that visualization could be an important part of the story. I like that suggestion.
I agree, Brian. Already for banner images accompanying popular summaries I’ve seen appropriate images (Red-winged Blackbirds, Dark-eyed Juncos, House Sparrow, European Starling), less appropriate images (Baltimore Orioles), and widely inappropriate images (e.g. Snowy Owls). But in my Bird Conservation Region (Prairie Hardwood Transition), BBS surveyors saw or heard an average of ~ 3 Western Meadowlarks per stop back in 1967. Today you have to do 3 entire BBS routes (@ 50 stops) to pick up 1 WEME (and it’s not like Eastern Meadowlarks have picked up the slack). I’m not in favor of sensationalism, but I’m going to relish the added attention to concerning ourselves with declines while things are still abundant. Your skepticism and criticism are highly welcomed, as always.
Thank you for this analysis
Was going to post a somewhat glib comment and thank you, but on second thought, there is so much here to digest.
Just thanks. I’ll withhold the glib comment. As a part-time nature journalist years back, it has me thinking about how much I probably oversimplified, what wrong impressions I may have given.
Don’t get me wrong. There is plenty of great nature journalism out there. And it is super important. And I expect not easy. We scientists usually get to spend months thinking about and tuning our message. Where as journalists often have deadlines in hours or at best days.
Thanks for stopping by.
Very thorough and interesting critique. You bring up a lot of great points. I’d like to argue though for the value of ‘sensationalist’ statements like the 3 billion birds statistic. First off, I think most of us would agree that bird conservation (and conservation in general) is more limited by a lack of awareness and political motivation than by a lack of data. I’ve seen more popular news articles about birds in the last day than I have in probably the whole last year. And clearly it’s already stimulating great conversation among ecologists too! (Kudos to Brian for the very quick and thorough response). Second, although the 3 billion birds statistic is clearly a broad generalization it does highlight the fact that common species are declining more than we typically realize. Yes, many of those common species are declining from human-caused population booms, but other species aren’t picking up the slack. This suggests a fairly drastic decline in continent-wide avian biomass, which is likely to impact ecosystem function much more than the decline or even extinction of a few rare species.
As a scientist I wish there were some really good replicated experiments on whether 3 billion motivates more or less policy action and behavioral change than a more nuanced discussion. I kind of suspect the latter but I can’t prove it. And I know reasonable people can suspect the other.
There is no way around the fact that this study shows there is a real and systematic decline in birds. That is important to know.
Thanks great to see your post. I’ve been working on an article to help people scared by the story – I’ve focused on the counting method, whether the numbers could have been biased. For instance most of it is based on 25 mile road counts stopping every half mile to count all the birds seen for three minutes. Road traffic has increased hugely since the 1970s. How much of the reduction could be just a reduction of the numbers of birds within a few miles of roads due to traffic fatalities? Also cat ownership has increased hugely – so how much is due to mortality from cats, if it is a real reduction.
I’ve also focused on the extinction argument which I found very bizarre for a peer reviewed article. They motivate the idea that reductions of populations of very numerous birds could be a sign of extinction by using the analogy of the passenger pigeon. But the passenger pigeon was such a very special case where they continued to hunt and hunt more intensively as the numbers dropped, with no idea that it even needed to be conserved. A hobbyist could have saved the species by keeping a small flock of birds in his backyard but nobody even thought of doing that.
I’ve referred to your blog and quoted you part way through, and interested in any comments. I am not an ecologist, I’m a mathematician by training and a software developer for my own programs – but I have been science blogging for a while, I did postgraduate maths, learnt how to read scientific papers and how to check citations etc, and my maths background also helps to see through arguments from pathos, indeed my postgraudate speciality was in logic, which helps with seeing through weak arguments in papers.
The paper didn’t manage to convince me that there even was a reduction in numbers at all. I think there probably is, e.g. for grassland species, from another paper but if this was the only evidence I would not have been convinced by it. Rather – it highlighted the difficulties in such methods.
Any thoughs on any of what I say do say, thanks!
I am very familiar with the Breeding Bird Survey (BBS) that was used to derive the trends. No survey is perfect. But the BBS is one of our best inventory and monitoring protocols anywhere in the world. It was setup by careful scientists to ask exactly these questions of relative abundance (which species are trending up or down, which species are more common or rare, what are overall trends). And it has been run for 48 years with thousands of routes. It has high quality observers and uses quality flags on runs. Any survey is going to have biases. And the road biases you mention is the most common critique of the BBS. But having walked some BBS routes I can say: a) some of those roads are dirt roads in national conservation lands, and b) roads are an ever present and growing feature of our landscape. But nobody has really shown in any quantitative fashion that I am aware that the BBS is systematically biased. There are some known biases of the BBS it doesn’t sample water birds or night-active birds well. But these are well known and taken into account (among other things there are some supplementary programs that survey these animals better).
Bottom line, I would bet a lot of money on the relative trends that come out of the BBS being accurate. I think decline in total number of birds in NA is real. I would put much less faith in the precision of assigning a 3,000,000,000 number to that. And I think looking overall it hides a lot of variability. As noted in another comment -6.6 billion loss and 3.3 billion gain is a lot of churn or turnover. So we need to understand that turnover. And we need to understand who is winning and who is losing (some of the losers I’m not so sad to see losing).
But losing a non-trivial chunk of our total birds is (as I’ve said several place on this page) is I think all too real.
Hi Brian, Sorry for the delay in this reply. Thanks so much for explaining. I’ve edited my article and added a quote of this comment to the section about the observation methods. I have left the section in since other readers might well, like me, be interested to know a bit more about how reliable their observation methods are.
Thank God for this article. Sensationalism is why I don’t care about the “news” anymore. I wish more people used their critical thinking skills.
Good work! In addition, if you look closely at the data, the great majority of the decline happened before 2000. Counting habitat groups, 40% are stable or increasing since 2000. Everything declined Pre 2000.
Thanks for a great post.
Great post. Thank you.
I was struck by the numbers which seemed to indicate the problem centers on migratory birds. Resident birds have seen population increase (no?). What is the migration? How many of these migrate to/from central and south america? How many to the far north? Where is the drop off occurring? These are N American counts, but where is the issue?
I haven’t looked at this closely in this paper yet, but yes I think you are correct that migratory songbirds are showing most of the declines – which might be more about their subtropical/tropical winter habitat (or their migratory corridors).
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In 2016 we published an analysis of the North American bird population trends 1970-2010 using the BBS data (https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.13292) in GCB. We found:
1. Decreases in total abundance (but not nearly as large as 30%)
2. Strong decreases in grassland species but increases in forest species
3. The species driving the decreases are the most abundant species (75%-100% abundance quartile)
Unfortunately, our study has gone mostly unnoticed. Rosenberg’s et al inference for the total abundance change in minus 3 billion birds is novel and powerful. But I do think that there maybe a bias in the -30% average declines they report (and your nice analysis, Brian, starts to uncover that) and a lack of emphasis on the species/species groups that are increasing.
That is a nice paper (I followed it when it came out). It has the very important message of abundance and diversity being decoupled (and also the compositional change theme that I have hit in this blog post).
Hi Brian. Nice job on bringing some of the numbers from the supplement to the light of day. Just wanted to plug in a blog post I wrote, hinting that the 3B number might actually be an underestimate, based on studies we have been conducting in the north: http://blog.abmi.ca/2019/09/23/bird-declines-in-north-america-a-deeper-dive-using-long-term-abmi-and-bam-data/
Nice to know about the BAM and ABMI datasets. They (especially ABMI) look like what we need to be doing globally, not just Alberta.
There is clearly a boreal bird decline signal that is 2nd to grassland birds. But I confess I’m not as clear on the exact mechanisms as with grassland. It is not just or even primarily old growth/mature forest boreal species taking the hit.
Hi Brian, I really appreciate your approach to explaining this paper. However, I would like to quibble a bit (as I think that you invite us to do) with your take on how seriously we should take the decline of common birds. ‘Keeping common birds common’ has been a rallying cry for conservation organizations going back decades (see Partners in Flight), not only because of historical precedents like the extinction of the Passenger Pigeon, but also because it is far more cost effective to keep common birds common than it is to try to save a species once it is on the brink of extinction. Just think of all of the literal millions of dollars that have gone into saving the California Condor that could have been focused elsewhere. (And while condor numbers are increasing, they are nowhere near being self sufficient as a species.) Furthermore, as you note, many of the species that have declined most are species that benefited from the spread of low-intensity agriculture and may now be declining back to baseline levels. This is likely entirely true. However, what has transpired in Europe–where the intensification of agriculture has occurred in some countries at a far more rapid rate than in the US–is that many similar species just kept right on declining past their historic, pre-agriculture, baseline levels. For instance, we have estimated that there were ~40,000 breeding pairs of black-tailed godwits in The Netherlands in 1880, 120,000 pairs (as a result of the spread of low-intensity agriculture) in 1960, and fewer than 20,000 now (see Kentie et al. 2016, Ardea). For such species, where and how do we put a stop to this? If it were merely a matter of forest birds all recovering and agricultural birds declining to baseline levels that would be one thing, but the reality is that for many of these species, the natural habitats upon which they relied pre-agriculture are now gone, and so low-intensity agriculture is all that they have. Finally, as for boreal forest breeders, I would argue that, in fact, there has been fairly widespread recognition that many of these species (and especially aerial insectivores) are in serious decline (see Nebel et al. 2010, Avian Cons. Ecol.). And, yes, while Blackpoll Warblers and Dark-eyed Juncos do nest in edge habitats across the boreal forest, they also nest in many other types of boreal forest than may not have been so directly altered by humans. I would therefore hesitate to argue that these species are also simply regressing to the mean, so to speak. Thus, while I share your concern about the sensationalist nature of much press coverage, we shouldn’t look away from the fact that large scale declines in common species is a very troubling prospect, whatever the cause.
Hi Nathan – by all means quibble or disagree!
Certainly the boreal bird story is not particular well understood by me, or I think anybody. Peter Solymos (comment just above) links to a boreal bird survey I hadn’t heard about it and suggests it is worse than the BBS suggests.
The conservation concerns about declining abundant birds is certainly not black or white. Nobody would say they are the top priority I think. But they are certainly a canary in the coal mine that things could get a whole lot worse. But on the other hand we have to recognize the increases as well.
i didn’t know the Netherlands godwit story so thanks for sharing that with me.
I think the problem is that the broad-based decline of the common species (as opposed to the specific examples of invasive species, etc) is that anything that sweeping and systemic has to be a sign of something that is sweeping and systemic – primarily human land appropriation. How are we going to reverse that? In many way it is a much harder problem than normal conservation questions. I think if people are going to raise the issue of common species declining we have to be honest with the public about the cause and be willing to take on that issue. People seem much more interested in getting rid of plastic straws than tackling the degree of land appropriation in modern society.
Recovering habitat is easy, just take back the 35 million acres wasted on idiot ethanol
I totally agree that the decline of common birds is a sign of a sweeping and systemic problem! We are definitely in need of creative and feasible conservation measures that start to stem the tide of habitat loss. There are some bright spots (https://www.scienceforconservation.org/science-in-action/birdreturns) that we just need to find ways to scale up…
I agree – I really appreciate your thinking on this, and I like your perspective too. But aren’t you slightly misinterpreting the purpose of this paper, and of the 3B estimate? You’ve framed your response in the context of species extinctions (hence the importance of not letting rare species’ changes be swamped by common species), but the authors specifically frame the article in contrast to that question. They say in the abstract that: “Species extinctions have defined the global biodiversity crisis, but extinction begins with loss in abundance of individuals that can result in compositional and functional changes of ecosystems”. Their introduction starts with: “The overwhelming focus on species extinctions, however, has underestimated the extent and consequences of biotic change, by ignoring the loss of abundance within still-common species and in aggregate across large species assemblages”
They’re therefore interested in changes to avifaunal biomass, and to “the associated loss of ecosystem integrity, function and services”. From that perspective, it’s fine for massive declines in common species to mask increases in Californian Condor. It’s even a problem if the abundance of an invasive species declines, if they’re providing some essential service or function, and not being replaced by a functionally-equivalent biomass of other species.
You also say that: “The conservation concerns about declining abundant birds is certainly not black or white. Nobody would say they are the top priority I think.” I don’t know about that. The Amazon is full of abundant tree species that are at no risk of extinction, certainly relative to other species. Our concern is for the local and global ecosystem services they provide via their biomass. Similarly, societal interest in wildlife isn’t particularly centred on rarity – if you don’t ask conservation scientists or twitchers, people seem to enjoy an abundance of colourful fauna against a green (or blue, if marine) backdrop.
I agree biomass is important in its own right. And the authors were careful. But did you notice how many articles covering this also mentioned a 6th mass extinction?
It’s probably a reflection of the sources I read (the internet in a nutshell), but they’ve basically steered clear of mass extinction statements, and they always mentioned the nuance (that some birds have increased).
Glad to hear it. I hope I’m wrong on how common the linkage is. Its just anecdotal but two of the first three articles I saw Thursday night did mention it.
I don’t think we’re that far apart in what we’re saying.
But I’m trying to better understand what you’re getting at. In a world of finite resources and finite social willingness to change behavior what do you think this paper leads to society changing from current practices (mostly focusing on rare species, preserving land to the extent economically feasible, various laws like clean air, clean water, endangered species)?
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Thanks for the excellent post, Brian. Ecology needs more thoughtful interpretations like yours that put numbers in their appropriate context.
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As the provided link to The Wire story shows (https://thewire.in/environment/in-a-new-study-on-bird-loss-some-scientists-say-subtlety-is-lost-too) even the subtlety of your critique of the Science article is easily lost. Instead we find here a writer selecting quotes from you that bolster the narrative that scientists are not to be trusted. This narrative falls as music on the ears of many whose reasons range from a dislike of birds to strict devotion to rationalism to worship of humanism to evangelical faith in the divine.
Of course we have seen this before, most notably with climate denial. Whether you intended to or not, you just stepped into the role of “bird decline skeptic.” When I read through the entirety of your remarks and replies in the Dynamic Ecology piece here, I see plenty of places where you hedge away from that position, but, these days, most ears are not tuned to hear subtlety. These days, short of grabbing people by the throat, there seems to be no other effective way of waking people up to the fadeout of the natural world.
For those of us who attempt to pay attention to nature, the Science article was more affirming than surprising. Talk to any birder, fisherman, botanist or naturalist and you hear the same lament. Where have our beloved gone? In your Dynamic Ecology you do highlight the problem: “So are humans impacting birds in a negative fashion overall or on average. Pretty evidently yes. The Anthropocene is real. It’s hard to take away 50% of the total land and 50% of the freshwater each year and 50% of the new plant growth each year globally and not see declines! Given those statistics its actually kind of surprising the declines are not more.”
But you pull the punch in the next sentence, in what is sure to get more airtime from skeptics of bird decline: “And in a small good news embedded in larger bad news fashion, most of those 3 billion in declines are in species that are not at even the slightest risk of extinction.”
There is no such thing as unbiased science. While you attempt to take refuge behind your own impartiality, your true biases shine brightly through. It seems that you love science more than birds. Fair enough, but full disclosure is warranted.
I didn’t take the Undark piece that is the source for the link you provide as anti-scientist at all. Their piece has been picked up and modified several times so I’ve only skimmed the The Wire version you link to. What I see is a recounting of scientists having a robust conversation with each other about what data means. That is not anti-science. That is pro-science.
Scientists can agree on the basics and disagree on the details and interpretations. Happens in climate science too. Doesn’t make whoever is on the side against the more doomed scenario anti-science tough!
I don’t think I pulled a punch in the sentence you quote. I draw a very important distinction between decline in abundance/biomass and loss of species/extinction. Those can be linked in fact in some circumstances, but are almost certainly linked in the casual readers mind. And they really aren’t linked in this data. Being clear about that this data does not support an assumption that many people will make based on headline reporting of this data is a question of accuracy and honesty.
RE “It seems that you love science more than birds.” You’re overloading the word love here – two different kinds of love. And I do love both. But is my love of birds going to cause me to dissemble and mislead. Never. In that sense, I plead guilty to loving science more than birds.
RE “These days, short of grabbing people by the throat, there seems to be no other effective way of waking people up to the fadeout of the natural world.”
That I think is the heart of our disagreement. I believe treating the public with resepct as thoughtful people who can handle more detail than a headline is a winning strategy longrun for science and conservation. You think the opposite. Both of us are making a guess because there is no hard data proving one way or the other. I will only note that the “go for the jugular” approach to scaring people into stopping smoking didn’t work very well.
Cigarette packs now display a photo of a cancerous lung. And smoking is way down, as reported by the CDC. I’d call that an effective use of the jugular move.
Actually look at the time lines more carefully. The scare warnings on cigarette packs came a long time ago and very little movement on smoking followed. It was more recent advertising that was more social and more hopeful about the ability to change that most public health officials think was effective. There is a whole body of literature on what motivates people to take actual change to improve their health and scaring people does not look as effective as some other techniques.
Thanks so much for this article. As a policy nerd with significant boreal forest management experience (but almost no knowledge of birds) I have often wondered about bird population dynamics as they relate to forest age.
In my experience working in Canada’s boreal forest, at least that part of it that is managed for timber harvest, fire suppression over the past half century has created a forest age class structure that is unnaturally old. Significant area of forest has not burned and this has its own set of ecological consequences particularly as the climate changes which have been the focus of many policy discussions. Not however as they relate to birds to any large degree.
You mention the population changes in avian species due to shifts in forest age in eastern and central NA (as farmland shifts to forest). Are there any avian population studies you are aware of that look at the similar shift in age that has been happening at landscape levels in the boreal forest? I would be keen to read them if they are out there.
I am not aware of any studies (although I have not yet looked hard). My initial reads suggested that the boreal declines (which is probably the clearest 2ary theme after the grassland/farmland declines) are relatively unresearched in any large-scale or general sense (other than documenting the trend). I can find small scale detailed studies showing a specific mechanism has some effect and quite a bit of speculation, but little that rigorously compares multiple mechanisms over a region or extended period of time. Fire suppression is not high on the list of things studied. The authoritative Birds of North America compendium comes to a similar conclusion for most of the boreal birds I looked at (often suggesting they are poorly researched because they are so common).
Personally, I think your guess of fire suppression is a good one but that is not based in data. I also think the notion of migratory birds having trouble in their flyways or winter habitats is another good guess (not all boreal birds are migratory of course but many are).
Its also true that most of the data we have about boreal birds comes from the southern boreal forest (US and southern Canada) where the BBS is actively monitoring. The more northern boreal forest (obviously all in Canada) is less intensively studied so I have heard speculation both that the birds are doing better in the north and that they’re doing worse (Peter has links in a comment above suggesting they’re doing worse in the north). Pinning this down could also be suggestive of causes.
Also worth noting that similar trends are happening in Europe.
So I think the best answer I have is we don’t know. It highlights the importance of these large-scale monitoring projects and the analysis of their data (like the Science paper) to find the patterns, but it raises new questions.
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Hi All !
Did You really think that birds is decline on 33 % and after 100 years birds are all is died ?
If you were conducting field research you would not say so. There is no reduction; there is only improper research. And besides, YOUR mathematical calculations are the direct intellectual trap. If you imagine that there were 10 tits on the feeding trough 30 years ago, now there are 5 of them, and in some cases there are 2 or 3. However, this is not and cannot be in reality. Forests have not decreased over this time, the climate has become warmer, people are feeding more birds, and in general the environmental characteristics, at least in developed countries, are much better than 50 years ago. So all insinuations and your data are bad due to incorrect research methods. Return to the field gentlemen, scientists, and do not sit in the cabinets.
I do think there is something to what you say about needing to square what people (not scientists) are observing with scientific conclusions. I especially think this e.g. about the LPI which suggests a loss of 60% of vertebrates in 40 years which does not square with what people see.
But I think you mischaracterize the Breeding Bird Survey (and Christmas Bird Counts) which are very field based and very high quality.
I do not think that short-term observations of 1 to 2 weeks are able to characterize the state of a particular group of animals and even plants. And even more so, creatures moving in space and time.
Even if 30 percent of the land was studied at some brief point in time for the number of birds, this does not mean that in the same place (in the same area) in a month there will be a similar picture. An example from another area – spatial – research on orchids. Decades of research on 10 percent of the forests gave a picture of a sharp decrease in the number of this group of plants. After 25 years, something unusual happened by moving a bit of the spatial survey grid and got the opposite picture. Orchids of certain species do not die out, but rather flourish.
And I already know how bird counts are carried out by amateurs and how they are distributed in space. EVERYTHING REMAINS TO WISH THE BEST in all countries of the world.
All our problems in our head and how we used our time and perspective of science activities in animal and plant ecology.
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Brian, thank you for bringing up these points. Reminds me of the major public and professional kerfuffle, and ultimate scientific redressing, of the Worm et al. “no fish by 2048” article. My environmental science class is talking about “best science available” and advocacy debates today, and I’ll be asking them to read and think about the Science article and the responses posted here. Thanks again.
Yeah it’s the “no fish by 2048” angle that worries me the most. The authors didn’t go there. But I do think it is where most casual readers are going in their mind. And it seems like a rather unlikely outcome to me. And that is where I think scientist credibility gets at risk. Its in this very grey area where no scientist (in the case of birds) predicted eminent elimination of birds (although the phrase in the abstract “address threats to avert future avifaunal collapse” certainly plants the seeed), but we kind of walked people up to the brink of that conclusion and then let them take the last step themselves. So in theory scientists aren’t responsible for what people think. But when people still see a ton of birds at their feeders in 2050 (although maybe all Robins and grackles and invasive species) and start cracking jokes about “remember when scientists said all the birds are disappearing” its still going to undermine our credibility and goals just the same.
Sounds like a great class conversation.
This is a very astute comment.
This latest publication (or campaign?) reaches a new level of statistical and public relations skill. I really appreciated Brian’s respectful and thoughtful criticism, and I take the liberty to add a few observations.
3 billion birds less, that’s an impressive number and it is certainly a motivation to keep developing conservation solutions. But does it justify terms like collapse, and loss of ecosystem ‘health’ or ‘integrity’? Those terms have much more to do with politics than science.
Sticking to science, Brian raises the fundamental issue of temporal perspective, referring to the boom and bust of human-associated species over the last century or so. By beginning in the 1960s, the Breeding Bird Survey has set an arbitrary temporal reference that is bound to skew our perception of long-term phenomena. Imagine how our perception of birds’ ups and downs would change if we had bird population data well-before the colonization of the Americas by Europeans.
Rosenberg et al used the Passenger Pigeon example to point out that no species, however common, is impervious to the human enterprise. I question the legitimacy of this example, because thanks to the greater affluence of our societies, we now pay attention to the misfortunes of wildlife and we are able, more than ever before, to prevent extinctions. The relationship between population trends and species abundance showed in Brian’s post possibly results from that enlightenment. In 2004 the IUCN itself showed that the number of bird species extinctions has sharply declined in the last 50 years*. With advances in conservation science, I believe, for what it’s worth, that there is more room for optimism than fear.
Let me offer a few insights from my region, the province of Quebec. Birders here amassed nearly 10 million bird sightings since the 1960s. I combined those data with eBird, resulting in 1.57 million complete checklists, and found that the total number of birds observed per hour of birding has not changed significantly since 1970 (0.7 ± 0.6 more birds per hour per year). 116 species have declined over that period in the province, but 165 have increased (using a negative binomial regression of totals by year, offset by log hours of observation). Of course, this observer-centric perspective may not closely reflect actual bird numbers, and thus does not necessarily contradict the findings from that publication. But given the large (8x) increase in the numbers of contributing birders in Quebec between 1970-2019, it does show that never before have so many birds been observed (by so many quebecers).
In a perverse kind of way, for birders the net ‘value’ of species like common grackles may actually increase as they become scarcer. That’s because birders will not take them for granted as much as they may have done before. Some of the authors of this publication are keen birders, and they surely realize that birders generally do not seek hordes of grackles and starlings. Instead, they relish the diversity of species and seek out rarities. So I question the claim made in that publication that fewer common birds would hurt the social benefits from birds.
As for the “ecological integrity, function and services”, how are the observed changes in bird numbers supposed to affect these buzzwords? I have yet to find useful definitions of these terms beyond value judgments. The notion of ‘services’ is perhaps less subjective, but it remains extremely difficult to asses. For example, there are negative services rendered by some of the common species, such as nest or crop predation.
To sum up, I share Brian’s view that despite its obvious contribution to ornithological science, this paper may in the end contribute to unwarranted despair thanks to the coverage by media that was predictably alarmist. The recent trend in Science and Nature to publish papers blending politics and science should concern all of us who wish to keep cynicism to a minimum in the public’s perception of science.
* Figure 3.2 in BAILLIE, J. E. M., HILTON-TAYLOR, C. and STUART, S. N. (eds.). 2004. 2004 IUCN Red List of Threatened Species. A Global Species Assessment, IUCN.
Thank you André. Some very important points in there. I find your checklist result very interesting. I recall working with it. It is an impressive data set.
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Dears Andre, Brian !
Good morning !
I once again see across data that used for prepared “scientific” papers about long-term bird population dynamic in North America and Europe too.
What I can said about it is not good data. Please look tables that contain data from some States or Province or Countries in 2018 years BBS data for North America, and in Europe from winter census reports of Sweden and Finland on my lovely species. I take areas similar by climatic and biome in wood.
Species Count birds
Black-capped Chickadee 349
Boreal Chickadee 3
Tufted Titmouse 2
Red-breasted Nuthatch 184
White-breasted Nuthatch 91
Brown Creeper 12
Black-capped Chickadee 280
Tufted Titmouse 67
Red-breasted Nuthatch 109
White-breasted Nuthatch 105
Brown Creeper 14
Black-capped Chickadee 980
Boreal Chickadee 18
Red-breasted Nuthatch 588
White-breasted Nuthatch 125
Brown Creeper 27
Parus major 5942
Cyanistes caeruleus 3311
Periparus ater 356
Poecile montanus 769
Certhia familiaris 200
Sitta europaea 1340
Aegithalos caudatus 347
Mean numbers bird on 40 years
Parus major 29040
Cyanistes caeruleus 12796
Periparus ater 845
Poecile montanus 4500
Certhia familiaris 418
Aegithalos caudatus 555
The First – I specifically cite these figures in order to understand how it is possible to get a picture in the boreal and pre-boreal zones that non-boreal species are much more numerous than boreal ones.
The Second – I will never believe that in such large areas so few birds are counted. For example -treecreepers. This leads to such pathos articles about extinction.
The third – bird should not be counted over the heads, but the density should be calculated.
In yesterday in 17 km of routes (plot = 30 sq.km) –
I am censused – 228 Willow tit, 320 of Coal Tit,
36 treecreeper, 517 Long-tailed tit and etc. Please compared with above table.
I think your last point is key – catch/observations per unit effort. The different bird surveys you cite use very different approaches. The North American BBS never claims that there are a certain number of birds in any large area (even though the transects are long) because everybody knows their methods. One if its strengths is the observation effort is standardized across the years. So while one should not make claims about total number of birds from the BBS (a point I also made), I do think the trends from the BBS are reasonably reliable (albeit certainly there is some statistical noise – no trend can be estimated with infinite precision unless the entire population is counted every year – a logistical impossibility).
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Excellent article! I attended a talk about gulls where data was presented that showed a significant decline in gull populations in my area. At first the data seemed alarming, but the presenter then showed how gull populations had been significantly increased due to open landfills. Now that waste management practices are changing and open landfills are going away, gull populations are experiencing a “correction”. No environmental crisis was involved, in fact, it was to do better environmental practices.
The Science paper raised some questions for me.
First, is it even valid to sum up across all bird species? After >65 MY of divergence, do sparrows, hummingbirds, cormorants, and loons really have much in common any more? Do they live in similar habitats, eat the same food, have a common physiology, or share the same habits? If the answer is no, then any changes in the sum can’t be attributed to any general causes. And if they’re the sum of many separate causes that apply to different groups, then a priori the changes in the sum are due to the trends of the most populous species and habitats. Which as you say are just the ones that raise the fewest concerns.
Second, if we do accept that this sum-f-all-birds is a meaningful statistic, we have to remember that the instantaneous size of a bird species population is much more volatile than that of humans: most birds survive for only a few years. Say in April, 1 M black-and-white warblers reach their breeding territories. By June 1st, their population will expand to ~6-8 M individuals, and then subsides over the year in large part due to heavy juvenile mortality. The true sum of all individual birds must fluctuate annually by much more than 3 B individuals!
This may seem to be a trivial point since annual cycles certainly don’t preclude the existence of longer-term secular trends. But the possibility of sharp annual changes with very low autocorrelation raises statistical concerns. Consider that in figure 2 of the Sience paper there is a significant negative trend in eastern NA over 2007-2016. On closer inspection, this trend is driven by very high numbers in 2009-2010. These ‘highs’ could be just short-term expansions driven by unusually low juvenile mortality which would naturally ‘regress to the mean’ over the next year or two. Take them a way and the trend in pretty flat.
Lastly, it certainly isn’t true that bird populations have been crashing unnoticed. From my reading, the status of many if not all bird populations are closely monitored and most steep declines have well-understood causes. For example, the Evening Grosbeak was a huge beneficiary of spruce bud outbreaks in eastern Canada, but now that these have been suppressed, the Grosbeaks are increasingly restricted to their original western range. As mentioned above introduced species may be declining as local predators and parasites learn to switch to these new prey/hosts.
It’s important to focus of these specific causal linkages because this knowledge empowers people to take specific, effective actions and feel good when they succeed. Millions of people building birdhouses brought the Eastern Bluebird back. Better hayfield management can help the larks, bobolinks, sparrows, and blackbirds recover, or at least hold their ground. Broad-brush pessimism won’t.
All good points that I agree with.
On the 2nd point the surveys they used were run on close to the same day every year so in theory they capture the populations at the same point in the breeding cycle. But as you point out the differences across a breeding cycle are huge and even if the survey is run on the same day, year-to-year climate variability may change the exact point in the cycle you are catching.
The more I have thought about this, I think your point about understanding mechanisms leading to practical management outcomes is right on target.
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