Despite its dramatic-sounding conclusions, UK research into the “sub-lethal effects” of cats reveals very little about real-world predator-prey dynamics or their potential impact on bird populations.
Common blackbird. Photo courtesy of Wikimedia Commons and Andreas Trepte.
“A new study from British scientists has documented for the first time, significant new impacts to birds from outdoor cats,” announced an April 18 news release from the American Bird Conservancy, “reporting that even brief appearances of cats near avian nest sites leads to at least a doubling in lethal nest predation of eggs and young birds by third-party animals.” The study, by PhD student Colin Bonnington, Kevin J. Gaston, professor of biodiversity and conservation at the University of Exeter, and Karl L. Evans, conservation biology lecturer at the University of Sheffield (and Bonnington’s PhD advisor), was published earlier this year in the Journal of Applied Ecology as “Fearing the feline: domestic cats reduce avian fecundity through trait-mediated indirect effects that increase nest predation by other species.”
How the Research was Conducted
Researchers began by locating blackbird nests in urban Sheffield, England, during the 2010 and 2011 breeding seasons (March–August). They then monitored the behavior of nesting parents following the presentation, during the first year, of three different taxidermy mounts: an adult cat, a grey squirrel, and an adult rabbit “as a non-predator control.”  During 2011, the researchers used two “models”—to use their terminology—of each species, including “a replica adult domestic cat.”  (More on that “replica” shortly.)
Models were placed on the ground, approximately six feet away from the location of each blackbird nest (each of which was 0.8–6.6 ft. off the ground) during one of three stages of chick development: egg, young chick (1–4 days), or old chick (over 8 days). In each case, all three models were presented, in random order, over a three-day period. Each model presentation lasted 15 minutes, with a 24-hour interval between presentations. During 2010, researchers studied a total of 47 nests (17 at the egg stage, 15 at the young chick, and 15 at the old chick stage); in 2011, they studied 49 nests (20 at the egg stage, 15 at the young chick, and 14 at the old chick stage).
During each model presentation, the researchers recorded parental behavior “from a concealed location [49–98 feet] away, paying particular attention to alarm calling and aggressive behavior (e.g., “striking, diving towards or hovering within [6.6 ft.] of it”). Following the model’s removal, “provisioning visits” were observed for an additional 45–60 minutes.
What the Research Revealed
“The domestic cat model consistently elicited significantly higher alarm calling rates,” explain Bonnington et al., compared to the rabbit model, and in many cases, the squirrel model.  Aggression toward the model varied with the chick development stage. During the egg stage, blackbird parents responded similarly to each of the models; during the young chick stage, however, “aggression was higher in response to the domestic cat than the squirrel or rabbit, but differences were not significant,” and “when old chicks were in the nest, aggression towards the domestic cat was significantly higher, by an order of magnitude, than towards the rabbit and grey squirrel.” 
Following exposure to the cat model, blackbird parents reduced provisioning rates (as measured by the number of food-bearing visits to the nest each hour) by more than 33 percent relative to the rabbit model. Exposure to the squirrel model also resulted in a reduced provisioning rate, but to a lesser degree. “And,” note the authors, “there was no evidence that provisioning rates returned to normal 90 minutes after removal of the domestic cat model.” 
Sixteen nests were predated following model presentation. Here, too, differences were observed relative to both the chick development stage and the model used. During the egg stage, exposure to the cat model resulted in a 23 percent predation rate, compared to 5 percent for the squirrel model and none for the rabbit model. During the young chick stage, 13 percent of nests exposed to the cat model were predated while none of the nests exposed to either the squirrel or rabbit models was predated. And during the old chick stage, none of the nests were predated, “presumably because at this stage nestlings are sufficiently well developed to disperse rapidly from the nest site when threatened by a predator.” 
The Official Talking Points
Among the researchers’ far-reaching conclusions:
- “We conducted controlled model presentation experiments at active urban blackbird nests to provide the first empirical evidence that quantifies the potential sub-lethal and indirect effects of predators (domestic cat and grey squirrel) on avian reproductive success.
- “The brief presence of a domestic cat at avian nest sites reduces subsequent provisioning rates and induces lethal trait-mediated indirect effects.* We provide the first robust evidence for these latter effects in any avian predator–prey system, although they are likely to be common in many avian assemblages with high predator densities.”
A Closer Look
“Here we have yet another peer-reviewed study that documents additional, serious impacts to bird populations that previously have not been fully appreciated,” proclaimed Clare Nielsen, ABC’s director of communications in that April news release. “Feral and outdoor cats are simply devastating populations of birds and other wildlife.”
Is ABC really suggesting that dead cats are “devastating populations of birds and other wildlife”? After all, we’re talking about taxidermy mounts here.
Correction: one taxidermy mount of a domestic cat.
The “replica” added to the study during the second year, was, as Karl Evans explained to me via e-mail, actually “a black and white stuffed cuddly toy cat.”
Not that I considered the work to be particularly rigorous before I learned of this detail (one ABC either didn’t look into or didn’t feel important enough to publicize). As I explained to Evans, I think the study’s highly questionable design precludes the sort of conclusions he and his co-authors (and ABC) make. My objection is based largely on two key factors: (1) the artificial nature of the investigation, and (2) its brief duration.
- The conclusions being drawn here assume that the blackbirds actually interpreted the “model cats” as if they were live cats—and therefore, that their behaviors accurately reflect the real-world behaviors of urban blackbirds in the presence of cats. Which strikes me as highly implausible—in part because it’s difficult to imagine any cat remaining stone-still for 15 minutes** just a few meters away from a blackbird nest, especially once the parents have become so active.
- Even setting aside the artificial nature of the study, though, it seems to me that the outcome of any single nesting attempt says nothing about the population dynamics of the species involved. What if, as a result of the failures Bonnington et al. documented, the blackbirds’ next attempt was in a more secure location? Or if, over time, they become less fearful and therefore behave in ways that will reduce the likelihood of such failures in the future? Certainly there’s evidence of bird species as “urban winners.” Indeed, Anders Møller, whose research into the fear response of birds is extensive, suggests that “individuals from urban populations [are] less fearful of humans than their rural ancestors.” And, Møller continues: “Such reduced fearfulness among urban birds may have as a consequence that they are less disturbed by humans and their domesticated animals (mainly cats and dogs).” 
In the end, I simply don’t see that placing taxidermy models or plush toys near bird nests is going to tell us much about the population dynamics of a particular species. (In fact, I’m rather surprised the work was funded by the UK’s Natural Environment Research Council, which, according to the agency’s website, “coordinate[s] some of the world’s most exciting research projects.”)
However, such work can be used—as ABC has once again demonstrated—to fuel the long-standing witch-hunt against free-roaming cats. Although researchers are in no position to prevent their work from being co-opted in this way, they can certainly foresee the likelihood—and they therefore have an additional responsibility to produce only the most rigorous work and present only the strongest evidence.
Bonnington et al. simply didn’t fulfill that responsibility.
* “In addition to their sub-lethal effects,” explain Bonnington et al., “predators can determine prey population growth rates through other indirect mechanisms by altering prey species’ traits in a manner that changes their interactions with other species.” 
** Evans and his co-authors, referring to the work of Thomas Schaefer, suggest that this “is within the time range during which mammalian predators typically remain within the vicinity of birds’ nests,” But Schaefer was referring specifically to the time taken “to eat the nest contents.”  And although Schaefer did find that “mammals took significantly longer to eat the nest contents” than did avian predators, which “usually stayed just a few seconds at the nest to remove eggs or young,” it seems smaller mammals (e.g., mice) stayed longer than did larger mammals (e.g., martens).
1. Bonnington, C., Gaston, K.J., and Evans, K.L., “Fearing the feline: domestic cats reduce avian fecundity through trait-mediated indirect effects that increase nest predation by other species.” Journal of Applied Ecology. 2013. 50(1): p. 15–24. http://dx.doi.org/10.1111/1365-2664.12025
2. Møller, A.P., “Interspecific variation in fear responses predicts urbanization in birds.” Behavioral Ecology. 2010. 21(2): p. 365–371. http://beheco.oxfordjournals.org/content/21/2/365.abstract
3. Schaefer, T., “Video monitoring of shrub-nests reveals nest predators: Capsule Jays Garrulus glandarius are the most common predators, but carnivorous mammals and some other species also predate nests.” Bird Study. 2004. 51(2): p. 170–177. http://dx.doi.org/10.1080/00063650409461349