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Northern mockingbirdNorthern mockingbird. Photo courtesy of Wikimedia Commons and Ken Thomas.

“I thought the cats probably really hammered them when they were fledglings,” said former University of Florida doctoral student Christine Stracey in a press release about her study of Northern mockingbirds, “but when they were in the nests, I didn’t really expect the cats to be a huge problem. But I was really wrong about that.”

So, how big a problem are “the cats,” exactly?

No doubt it depends who you ask. But even Stracey, now an assistant professor of biology at Westminster College in Salt Lake City, seems to have rather different opinions on the matter.

In a paper documenting her research, to be published in Biological Conservation, Stracey readily acknowledges the limitations of her work:

“Before we can fully understand the role of nest predation in shaping urban bird communities, we need studies of a suite of species that do and do not thrive in urban environments and we need to study how predator diets change on a rural-urban gradient.” [1]

In the press release, however, Stracey’s not nearly so reserved: “We don’t see any reason why cats wouldn’t also eat cardinal nestlings, brown thrashers, towhees—anything else that is nesting in similar locations.” Noting that some of the cats caught on camera were wearing collars, she warns:

“People should not let their cats roam outdoors at all, but at the very least, keeping them inside at night will cut down on nest predation. Beyond that, we need to think hard about the feral cat problem.”

No wonder the story was picked up so enthusiastically by science sites (e.g., “To Kill a Mockingbird, Just Get a Cat” and “Cats No. 1 Predator to Urban Mockingbird Nests”).

While I agree completely that “we need to think hard about the feral cat problem,” I’m not convinced by Stracey’s work that cats are having any appreciable impact on the population of Gainesville’s Northern mockingbirds—never mind “anything else that is nesting in similar locations.” Indeed, Stracey herself describes the Northern mockingbird as an “urban winner,” and a “native species that is able to not only live with us, but do really well living with us.”

Well, which is it?

The Study
Stracey’s Biological Conservation paper combines the results of her work videotaping nest predators with data from 2005 and 2006, when she and her advisor, Scott Robinson, “documented [without the use of cameras] reduced nest predation in urban habitats at many of the same study sites.” [1]

The later work, conducted for her PhD thesis, involved collecting “data on nest predation rates at seven study sites (two parking lots, three residential neighborhoods, two pastures, and one wildlife preserve) in areas in and around Gainesville, FL between February and August of 2007–2008.” [1] In 2009, “one of the residential neighborhoods, one of the parking lots, and one of the pastures” were not included. [1]

Over this same period, Stracey used video cameras (clips are available at her Website) mounted at several nest locations (eight in 2007, 52 in 2008, and 84 in 2009) “to compare the identity of nest predators at nests in the urban matrix and non-urban habitats.” [1] Which, presumably, will shed light on the generally accepted urban nest predator paradox, which Stracey describes as the “mismatch between predation rates, which are often lower in urban areas, and predator abundance, which is often higher in urban areas.” [1]

In other words: if there are so many more predators is urban areas, why do predation levels tend to be lower?

Nest Survival
Using the logistic exposure method, Stracey calculated daily survival rates for Northern mockingbird nests for each habitat and each year of her study.

Here, the daily survival rate (DSR) is a measure of the odds that the eggs or nestlings will remain in the nest from one day to the next. A DSR of 90 percent, for example, means there’s a 90 percent chance—or nine-to-one odds—of a nest surviving from day to day. Or, to put it another way, there’s a 10 percent chance that the nest’s contents will be destroyed by a predator. (More generally, DSR includes any nest “failure,” but because Stracey was interested in predation, she “only considered nests that failed due to predation as ‘unsuccessful,’ thus her “daily survival rates… are not a measure of overall nest survival, but rather the probability of a nest escaping predation.’’ [1])

Nine-to-one odds sound great if you’re in Las Vegas, but no winning streak lasts forever. Over the course of the entire nest cycle, even a DSR of 96 or 97 percent takes its toll.

In the case of the Northern mockingbird, it takes about 13 days for the eggs to hatch and another 12 for the subsequent nestling stage. [2] (At least this is what Fischer found working in south Texas; Stracey doesn’t go into such details in her paper.) A DSR of 97 yields a 46.7 percent nest survival rate; a DSR of 96, only 36 percent (and this doesn’t take into account the additional “3 to 5 days for nest construction [and] 3 to 5 days for egg laying” observed by Fischer, who found that “a successful nest was used for an average of 33 days.” [2])

Assuming (conservatively) that there was essentially no risk of predation prior to the eggs being hatched, I used Stracey’s DSR data and a nest cycle of 25 days to estimate nest survival rates. (I also reordered the data to better compare habitats year-to-year.)

The result illustrates three important points:

  1. The residential neighborhoods—the only sites where cats were observed predating nests (more on that shortly)—are safer than either of the non-urban sites (the two pastures and the wildlife preserve) during 2005 and 2006, when cameras weren’t in use. (While I’m not suggesting that this difference is the result of the cameras, I’m not convinced that the cameras were as impartial as Stracey would have us believe—this, too, will be addressed shortly.) Indeed, even during 2007–2009, the results are, at best, mixed.
  2. The wildlife preserve, contrary to what its name suggests, fails to “outperform” the other habitats’ nest survival rates across all five years—sometimes (as in 2005, when the rate was just over 14 percent) spectacularly.
  3. Perhaps the most striking result, though, is the nest survival rates of Stracey’s two parking lot sites, which, in every year but 2008, top those of the other habitats—two to four times the rates documented at the wildlife preserve in some years. “Urban winner” is right. With apologies to Joni Mitchell, then, perhaps the best thing for the mockingbirds is to pave paradise and put up a parking lot.

Strong Nest Predators
My interest in Stracey’s work has less to do with nest survival itself, and much more to do with the role of cats in nest failure. Of the 24 predation events caught on tape in the residential neighborhoods, 17 (71 percent) were attributed to cats. By contrast, no cats were observed predating nests in the pastures or at the nature preserve. There, Cooper’s hawks were responsible for 15 of 33 (45 percent) predation events taking place at occupied nests. (One abandoned nest was raided by a blue jay.)

“I found no evidence,” writes Stracey, “that urban areas provide a refuge from strong nest predators.” [1]

“The strongest predator, cats, were essentially only found in urban areas, and Cooper’s hawks, the dominant predator in rural habitats, were found at roughly the same abundance in the two habitats (pers. obs.)… These results suggest that simple considerations of predator abundance do not explain habitat-specific patterns of nest predation.” [1]

Dig into the details of Stracey’s study, though, and it becomes clear that she’s probably overestimating the strength of cats as urban predators. In fact, her camera placement (intentionally or not) almost certainly biased her data.

Stracey didn’t use nest cameras in the parking lots “for fear of theft,” [1] a perfectly reasonable concession, but that means excluding approximately 36 percent (according to my calculations) of “urban” nest predation from her “whodunit” data set. And it’s easy to imagine that predation at these sites was skewed heavily toward predators other than cats (probably birds).

In terms of identifying individual predators, then, “urban” habitats include only residential neighborhoods.

Stracey also eliminated as candidates for video cameras 21 (7 percent) nests located more than 4 meters (approximately 13 feet) off the ground. These nests would seem to be largely inaccessible to cats. Similarly, she eliminated 18 of 124 (14.5 percent) of nests “located in trees that were right on the edge of a sidewalk or street that did not afford a place to hide the set-up.” [1] Again, perfectly understandable, but this almost certainly biased her results.

In short, it seems Stracey observed predation by cats largely because she placed the cameras where the cats were.

In addition to the 58 events caught on camera, “22 predation events were missed due to problems with the camera set-up, including drained batteries and damaged wires.” [1] Whether or not these incidents were evenly distributed across the three different habitats isn’t clear, and the implications could be significant.

As it is, the majority (58.6 percent) of videotaped predation occurred at non-urban sites; had the bulk of the technical problems (which accounted for 27.5 percent of the 80 predation events Stracey had the opportunity to videotape) also occurred at those sites, Stracey would have a more difficult time concluding, as she does, that “urban areas clearly do not provide a generalized refuge from nest predators.” [1]

Or maybe not. Even if Cooper’s hawks accounted for twice the nest predation as cats, I suspect the focus would merely shift from absolute predation rates to predation by non-native predators.

And finally there’s the question of whether the nest cameras had any effect on predation levels. Stracey says they didn’t, but it appears that she simply compared failure rates for nests with and without cameras for each year the cameras were in use.

Richardson et al. (whose work Stracey cites) point out that “expression of neophobia and wariness among larger mammals, corvids, and raptors is likely to differ according to the extent to which those predators have been persecuted [inline citations omitted]. Such variation should be considered when interpreting the results of camera studies from different habitats and among different groups of nest predators.” [3, emphasis mine]

“Although camera equipment at nests likely does attract the attention of generalist predators, we propose that many predators may not respond positively to these potentially novel objects. Some small rodents, in particular, can be highly neophobic, reacting to novel stimuli with extreme caution and often avoidance (Barnett and Cowan 1976, Innes 1978). Though seemingly fearless and inquisitive, wild corvids likewise can be highly neophobic, particularly toward objects that do not resemble natural food items (Coppinger 1969, Heinrich et al. 1995), which may explain why Thompson et al. (1999), expecting to document nest predation by American crows and blue jays based on their abundance at the study site and their reputation as nest predators, did not record predation by any corvids. Raptors may exhibit a degree of neophobia as well (Ruggiero et al. 1979). Both canids and corvids have demonstrated aversion toward remote camera equipment, possibly driven more by wariness of human activity than generalized neophobia (Hernandez et al. 1997, Harris and Knowlton 2001, Herranz et al. 2002, Sequin et al. 2003). For nest studies using remote cameras, neophobia may be reinforced by the fact that camera equipment is seldom left in place for >2 weeks. As nests fail or fledge, camera equipment is relocated to active nests at new locations.” [3]

To illustrate what they mean by persecution, Richardson et al. cite the work of Herranz et al. (one of those in-line citations omitted above): “In our study area, Black-billed Magpies are shot and trapped by hunters, and this may increase their awareness of strange objects in the environment.” [4]

It seems reasonable, then, to expect that cats—the most “welcome” (especially those kept as pets) of the predators in Stracey’s study—would also be the least neophobic. (Though, of course, from a historical perspective, cats are arguably the most persecuted of the lot.)

It may well be, then, that Stracey caught more cats on camera largely because (1) the cameras were located—literally—in the cats’ backyards, and (2) the cats weren’t put off by the equipment.

•     •     •

I asked Stracey about all of this by way of e-mail, but received no response. My follow-up e-mail also went unanswered, but I did notice some Website traffic from the Salt Lake City area that same day. Of course, it could be nothing more than coincidence.

The same can be said for the traffic I saw from Columbus, OH, late last week on the same day I contacted Dr. Amanda Rodewald, professor of wildlife ecology at Ohio State University. From what I can tell, Rodewald had nothing to do with Stracey’s mockingbird study; nevertheless, she was quoted in the UF press release:

“There are a lot of loud voices that deny cats are important predators of birds in our cities… But this study shows clearly that cats were the dominant predator in this Florida system—and that wasn’t presumed, it was recorded on video, so it was fact.”

So, I sent Rodewald an e-mail—identifying myself as one of those “loud voices.” I explained that I wasn’t asking her to speak for Stracey, nor to defend the research. But, given her own research interest—and her obvious concern with Stracey’s work—perhaps she might be able to answer one question for me: What impact might we expect on the area’s Northern mockingbird populations if the cats were removed from the environment?

No response yet—though it’s only been a few days. Still, I’m not holding my breath.

Literature Cited
1.  Stracey, C.M., “Resolving the urban nest predator paradox: The role of alternative foods for nest predators.” Biological Conservation. 2011. In Press, Corrected Proof.

2. Fischer, D.H., “Factors Affecting the Reproductive Success of the Northern Mockingbird in South Texas.” The Southwestern Naturalist. 1981. 26(3): p. 289–293.

3. Richardson, T.W., Gardali, T., and Jenkins, S.H., “Review and Meta-Analysis of Camera Effects on Avian Nest Success.” Journal of Wildlife Management. 2009. 73(2): p. 287–293.

4. Herranz, J., Yanes, M., and Suárez, F., “Does photo-monitoring affect nest predation? Journal of Field Ornithology. 2002. 73(1): p. 97–101.[0097:DPMANP]2.0.CO;2

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