The third in a series of posts that breaks down my critique of the essay “Critical Assessment of Claims Regarding Management of Feral Cats by Trap-Neuter-Return” (Conservation Biology, Volume 23, No. 4, 887–894) by Travis Longcore, Catherine Rich, and Lauren M. Sullivan.
As I’ve suggested previously, much of the “evidence” that Longcore et al. cite regarding cat predation is rather weak. The links between predation and declining wildlife populations (especially birds) are, not surprisingly, no better. According to Longcore et al., “Comparative field studies and population measurements illustrate the adverse effects of feral and free roaming cats on birds and other wildlife.” Let’s take a closer look at some of these studies…
In canyons in San Diego native bird diversity declined significantly with density of domestic cats (Crooks & Soulé 1999). —Longcore et al.
- Longcore fail to mention the density of people here—the study site was a “moderately sized fragment (~20 ha) [approximately 49 acres] bordered by 100 residences.” 
- It’s also not clear how sites such as this one, which the authors describe as, “undeveloped steep-sided canyons… habitat islands in an urban sea,” correspond to the environment overall. Don’t forget: when Longcore and the Urban Wildlands Group sued the City of Los Angeles last year, it was to put a stop to publicly supported TNR throughout the city.
In a comparative study in Alameda County, California, a site with a colony of feral cats had significantly fewer resident birds, fewer migrant birds, and fewer breeding birds than a control site without cats (Hawkins 1998). Ground-foraging species, notably California Quail (Calipepla californica) and California Thrashers (Toxostoma redivivum), were present at the control site but never observed at the site with cats. Native rodent density was drastically reduced at the site with cats, whereas exotic house mice (Mus musculus) were more common (Hawkins 1998). —Longcore et al.
Hawkins’ dissertation  is so problematic—and cited frequently enough as “evidence” of the impact of cats on wildlife—that it warrants a post of its own. Here, I’ll touch on just a few issues.
- Hawkins describes the “cat” and “no-cat” sites as being similar enough that a valid A-B comparison is appropriate. But a closer look at the study suggests otherwise. Much of the cat site bordered the park’s lake and marina, not far from a number of picnic sites. Hawkins notes that there were more people in the cat area, but doesn’t even admit to the possibility that their presence may have influenced the numbers of birds and rodents he observed there.
- In addition, the presence of pesticides may have played a role. According to a 2002 report (the earliest I was able to find) from the East Bay Regional Park District, “The focus of Lake Chabot’s weed control efforts are vegetation reduction within the two-acre overflow parking lot, picnic sites and firebreaks around park buildings, corp. yard, service yard, and the Lake Chabot classroom.”  Now, Hawkins’ fieldwork was done in 1995 and 1996, but if there was any pesticide use going on during the study period, it may have affected the results—especially if the pesticide was distributed differently across the two sites.
- Hawkins writes, “The preference of ground feeding birds for the no-cat treatment was striking; for example, California quail were seen almost daily in the no-cat area, whereas they were never seen in the cat area.” What’s more striking to me, though, is the fact that five of the nine ground-feeding species included in the study showed no preference for the cat or no-cat area—a point Hawkins downplays, and Longcore et al. ignore entirely.
- Finally, it’s not clear how this study constitutes an experiment at all. Hawkins chose two sites, one where cats were being fed, and another (approximately two miles away) where cats were not being fed. We have no idea what the cat area was actually like prior to the cats being fed there—Hawkins merely assumes the populations of birds and rodents would have been identical to those found at the no-cat site. We also have no idea what effect the feeding had—what if the cats were present but had to fend for themselves? And we don’t know if the cats were sterilized, or what impact that might have had on the study. In other words, Hawkins doesn’t actually know enough about what’s going on at the two sites to conclude, as he does, that “it is not prudent to manage for wildlife and allow cat feeding in the same parks.”
In Bristol, United Kingdom Baker et al. (2005) calculated that the predation rates by cats on 3 bird species in an urban area is high relative to annual productivity, which led the authors to suggest that the area under study may be a habitat sink. —Longcore et al.
- The authors’ method for calculating estimated predation rates is quite problematic. To begin with, they make the all-too-frequent mistake of using a simple average as a statistic to describe a highly skewed distribution (as the authors themselves note, “the majority of cats (51–74%) failed to return any prey” ), thus overestimating the average number of prey killed by each cat. This error is further exacerbated when Baker et al. multiply by 3.3, a factor “to account for the proportion of prey killed that was returned home.” This multiplier comes from a study plagued by very small sample sizes and its own dubious mathematics.  Nevertheless, Baker et al. seem to defend the study: “to the best of our knowledge, this is the only study that has managed to quantify this parameter.”
- Also, Longcore et al. fail to mention that Baker et al. concede an important point: “collectively, despite [cats] occurring at very high densities, the summed effects on prey populations appeared unlikely to affect population size for the majority of prey species.”
- In addition, a subsequent study (also conducted in Bristol) involving two of the authors, suggests that much of the predation observed was compensatory rather than additive. That is, many of these birds would have, for one reason or another, died anyhow, whether the cats were present or not. Specifically, the authors found that “birds killed by cats in this study had significantly lower fat and pectoral muscle mass scores than those killed by collisions.”  Baker at al. are cautious about these findings, suggesting, “the distinction between compensatory and additive mortality does… become increasingly redundant as the number of birds killed in a given area increases: where large numbers of prey are killed, predators would probably be killing a combination of individuals with poor and good long-term survival chances.” But once again, their estimates of birds killed by cats are inflated by a factor of 3.3, as described above—thereby raising doubts about any level of “redundancy.”
- Another study to investigate compensatory vs. additive predation was more conclusive. Møller and Erritzøe compared the average spleen mass of birds killed by cats to that of birds killed in collisions with windows found that, “small passerine birds falling prey to cats had spleens that were significantly smaller than those of conspecifics that died for other reasons,” concluding that the birds killed by cats “often have a poor health status.”  In other words, the birds killed by cats were among the population least likely to survive anyhow.
Clearly, this is a complex—not to mention contentious—issue. But Longcore et al. make it out to be remarkably straightforward. The “adverse effects of feral and free roaming cats on birds and other wildlife,” they seem to suggest, are widely accepted common knowledge.
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Then, too, there are those—and there are many—who have disputed the broad-brush claims about the impact of cat predation on wildlife. Consider, for example, the following studies:
- Biologist C.J. Mead, reviewing the deaths of “ringed” (banded) birds reported by the public, suggests that cats may be responsible for 6.2–31.3% of bird deaths. “Overall,” writes Mead, “it is clear that cat predation is a significant cause of death for most of the species examined.” But the relationship between bird deaths and population declines is complex. In fact, Mead concludes that “there is no clear evidence of cats threatening to harm the overall population level of any particular species… Indeed, cats have been kept as pets for many years and hundreds of generations of birds breeding in suburban and rural areas have had to contend with their predatory intentions.” 
- Mike Fitzgerald and Dennis Turner come to essentially the same conclusion. (Their contribution to The Domestic Cat: The Biology of its Behaviour is a must-read for anybody interested in the subject.) “We consider that we do not have enough information yet to attempt to estimate on average how many birds a cat kills each year,” write Fitzgerald and Turner. “And there are few, if any studies apart from island ones that actually demonstrate that cats have reduced bird populations.” 
- Martin, Twigg, and Robinson conclude more broadly, “it is not possible to make any inferences concerning the real impact of feral cats on prey populations from dietary studies.” 
Given the scope of their essay, perhaps it’s understandable that Longcore et al. were unable to cover each aspect of this rather broad topic in a comprehensive manner. Nevertheless, the amount of research into cat predation and declining bird populations—a critical piece of the puzzle, to be sure—that was omitted, glossed over, and/or misrepresented suggests a clear agenda. Read carefully, their essay comes across as not as scientific discourse, but as fodder for a marketing campaign. (And, given the number of wildlife conservation/bird advocacy groups that have posted it on their websites—not to mention the L.A. Superior court decision—it’s been effective.)
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Many proponents of TNR will acknowledge that the practice is not appropriate for all environments. Crooks and Soulé’s work in San Diego’s canyon country (cited previously), writes Ellen Perry Berkeley, “suggests to even the most ardent TNR advocate that such a landscape might not be the best place for TNR… The world is a complicated place.”  Gary J. Patronek, the former Director of the Center for Animals and Public Policy at the Cummings School of Veterinary Medicine at Tufts University, and one of the founders of the Hoarding of Animals Research Consortium, goes further: “the release of cats into an environment where they would impact endangered or threatened species, or even into wildlife preserves or refuges, is inexcusable.” 
This, I worry, is a slippery slope. Given the current nature of the cat debate, how long would it be before every alley, Dumpster, and abandoned property is deemed a wildlife preserve? And, given the number of endangered and threatened species—and their wide range—would there be any environment left for cats? Nevertheless, Patronek’s larger point is an important one:
I do not believe that this is being advocated by cat protectors who see urban, managed colonies as an imperfect but still preferable alternative to the euthanasia of healthy animals. Abandoned pet cats whose own habitat has been reduced to colonies, and the wild species endangered by clear-cutting or beachfront development, are casualties of the same callous disregard for the lives of animals. I see little justification for shifting the role of cats to that of scapegoat.
For Longcore and the Urban Wildlands Group (and others, too, of course), though, there’s simply no place for TNR anywhere. But there’s something even more troubling with their argument: they’re asking the wrong questions. As Patronek puts it so eloquently:
If the real objection to managed colonies is that it is unethical to put cats in a situation where they could potentially kill any wild creature, then the ethical issue should be debated on its own merits without burdening the discussion with highly speculative numerical estimates for either wildlife mortality or cat predation. Whittling down guesses or extrapolations from limited observations by a factor of 10 or even 100 does not make these estimates any more credible, and the fact that they are the best available data is not sufficient to justify their use when the consequence may be extermination for cats… What I find inconsistent in an otherwise scientific debate about biodiversity is how indictment of cats has been pursued almost in spite of the evidence, and without regard to the differential effects of cats in carefully selected, managed colonies, versus that of free-roaming pets, owned farm cats, or truly feral animals. Assessment of well-being for any is an imprecise and contentious process at best. Additional research is clearly needed concerning the welfare of these cats.
In a future post, I’ll get into the ways Longcore et al. would have us measure the success of TNR.
1. Crooks, K.R. and Soule, M.E., “Mesopredator release and avifaunal extinctions in a fragmented system.” Nature. 1999. 400(6744): p. 563.
2. Hawkins, C.C., Impact of a subsidized exotic predator on native biota: Effect of house cats (Felis catus) on California birds and rodents. 1998, Texas A&M University.
3. Brownfield, N.T., 2002 Annual Analysis of Pesticide Use East Bay Regional Park District. 2003, East Bay Regional Park District.
4. Baker, P.J., et al., “Impact of predation by domestic cats Felis catus in an urban area.” Mammal Review. 2005. 35(3/4): p. 302-312.
5. Kays, R.W. and DeWan, A.A., “Ecological impact of inside/outside house cats around a suburban nature preserve.” Animal Conservation. 2004. 7(3): p. 273-283.
6. Baker, P.J., et al., “Cats about town: is predation by free-ranging pet cats Felis catus likely to affect urban bird populations?“ Ibis. 2008. 150: p. 86-99.
7. Møller, A.P. and Erritzøe, J., “Predation against birds with low immunocompetence.” Oecologia. 2000. 122(4): p. 500-504.
8. Mead, C.J., “Ringed birds killed by cats.” Mammal Review. 1982. 12(4): p. 183-186.
9. Fitzgerald, B.M. and Turner, D.C., Hunting Behaviour of domestic cats and their impact on prey populations, in The Domestic Cat: The biology of its behaviour, D.C. Turner and P.P.G. Bateson, Editors. 2000, Cambridge University Press. p. 151–175.
10. Martin, G.R., Twigg, L.E., and Robinson, D.J., “Comparison of the Diet of Feral Cats From Rural and Pastoral Western Australia.” Wildlife Research. 1996. 23(4): p. 475-484.
11. Berkeley, E.P., TNR Past present and future: A history of the trap-neuter-return movement. 2004, Bethesda, MD: Alley Cat Allies.
12. Patronek, G.J., “Letter to Editor.” Journal of the American Veterinary Medical Association. 1996. 209(10): p. 1686–1687.