It’s difficult to imagine now—after a banner week of revelations, accusation, and obfuscations—but headlines earlier this month were dominated by dire warnings of a different kind: a looming ecological collapse as demonstrated by dramatic declines in North America’s birds. Coverage included stories in the New York Times, Washington Post, The Atlantic, National Geographic, Vox, Scientific American, and many other publications.
According to the reports, North American bird populations have declined by about 29 percent since 1970, a loss of roughly 3 billion birds.
Which leaves only about 7 billion birds. But just six years ago, researchers (some of which were also involved in this most recent study) published a paper estimating that outdoor cats kill 1.3–4.0 billion birds annually in the contiguous U.S. alone.
Are we really expected to believe that this one source of mortality (estimated only for the Lower 48 states, remember) is responsible for killing more than half of the continent’s bird population, year in and year out? Although numerous news accounts have referred to cats as contributors to the declines in bird abundance, I’ve yet to see one questioning this basic arithmetic (see Footnote 1).
The math becomes even more tenuous when we consider the millions of birds killed by other predators; collisions with buildings, wind turbines, and vehicles; hunters; pesticides; various diseases; and from impacts associated with habitat loss (“by far the greatest cause of bird population declines,” according to the 2014 State of the Birds Report), dramatic declines in insect populations, our worsening climate crisis, and more.
Not surprisingly, a review of “Decline of the North American avifauna,” published in the prestigious journal Science, reveals a slightly more complex story. In the supplementary materials that accompany the article, for example, the authors note that they “did not attempt to estimate the annual increase in population size due to the influences of reproductive output,” and point out that “post-breeding total population could increase as much as four to five times the size of the pre-breeding population size depending on recruitment success of young of the year.” 
Still, juveniles account for only about 25 percent of the mortalities used to arrive at that 1.3–4.0 billion birds estimate attributed to outdoor cats. (There are, in fact, numerous problems with the assumptions and analysis underlying these estimates, as I’ve documented previously.)
In addition, the population estimates recently reported in Science include birds from parts of North America not considered in the estimates of cat predation (e.g., wetlands, coasts, Arctic tundra, and boreal forests). Making the necessary adjustments results in an estimate of about 5.7 billion birds for those populations most relevant to the earlier cat predation estimates (see Footnote 2).
This does little to change the bottom line. Indeed, attempting to reconcile mortality estimates with population estimates, we’re led to believe that predation by outdoor cats accounts for 16.4–50.6 percent (median: 30.4) of bird populations across the parts of the country for which data are available (or, in the case of mortality estimates, have been extrapolated). And these mortality estimates, it’s worth remembering, have been described repeatedly by the researchers involved as “conservative.” [2–5]
As it happens, some of these same researchers have quantified the impacts of other mortality sources: collisions with buildings: 365–988 million birds;  collisions with vehicles: 89–340 million birds;  power lines: 8.9–68.6 million birds;  and wind turbines: 140,000–328,000 birds  (see Footnote 3).
Accounting for these losses, then, outdoor cats have only 4.3–5.3 billion birds on the landscape to begin with. And yet some of these same researchers are telling us—repeatedly, adamantly, and with increasing urgency—that these cats are killing up to 4.0 billion annually. And that this estimate is conservative.
How can all of these claims be true? And if they are all true, how is it possible that there are any birds left at all? Edwin Schrödinger himself couldn’t reconcile such a conundrum.
- Rosenberg, K.V.; Dokter, A.M.; Blancher, P.J.; Sauer, J.R.; Smith, A.C.; Smith, P.A.; Stanton, J.C.; Panjabi, A.; Helft, L.; Parr, M.; et al. Decline of the North American avifauna. Science 2019, eaaw1313.
- Loss, S.R.; Will, T.; Marra, P.P. The impact of free-ranging domestic cats on wildlife of the United States. Nature Communications 2013, 4.
- Loss, S.R.; Will, T.; Marra, P.P. The impact of free-ranging domestic cats on wildlife of the United States (Supplemental Information). Nature Communications 2013, 4.
- Marra, P.P.; Santella, C. Cat Wars: The Devastating Consequences of a Cuddly Killer; Princeton University Press, 2016;
- Loss, S.R.; Will, T.; Longcore, T.; Marra, P.P. Responding to misinformation and criticisms regarding United States cat predation estimates. Biological Invasions 2018, 20, 3385–3396.
- Loss, S.R.; Will, T.; Loss, S.S.; Marra, P.P. Bird-building collisions in the United States: Estimates of annual mortality and species vulnerability. The Condor 2014, 116, 8–23.
- Loss, S.; Will, T.; Marra, P. Estimation of bird-vehicle collision mortality on U.S. roads. The Journal of Wildlife Management 2014, 78, 763–771.
- Loss, S.R.; Will, T.; Marra, P.P. Refining Estimates of Bird Collision and Electrocution Mortality at Power Lines in the United States. PLOS ONE 2014, 9, e101565.
- Loss, S.R.; Will, T.; Marra, P.P. Estimates of bird collision mortality at wind facilities in the contiguous United States. Biological Conservation 2013, 168, 201–209.
- Gill, F.B. Ornithology; 3rd ed.; W.H. Freeman: New York, 2007; ISBN 978-0-7167-4983-7.
- Møller, A.P.; Erritzøe, J. Predation against birds with low immunocompetence. Oecologia 2000, 122, 500–504.
- Baker, P.J.; Molony, S.E.; Stone, E.; Cuthill, I.C.; Harris, S. Cats about town: Is predation by free-ranging pet cats Felis catus likely to affect urban bird populations? Ibis 2008, 150, 86–99.
(2) Two adjustments were made here: (1) breeding populations in wetlands, coasts, Arctic tundra, and boreal forests were excluded; and (2) the population of non-native species were included.
(3) It’s worth noting that these mortality sources seem to be largely indiscriminate—unlike predators (cats included), which tend to favor particularly vulnerable prey (e.g., young, old, sick, injured, etc.). [10–12]