Grey Catbirds and Predation

According to a recent news article, roaming cats kill up to a billion birds a year. The article then cites a recent study involving Grey Catbirds and predation. In my opinion, the article does a better job of interpreting the data and other related data on cats versus wildlife than other stories. But whenever I read a story that cites a study, I always find it a pleasurable and fun experience to read the study myself, if possible. Thankfully, this study is free and available for all to read.

The Gray Catbird
This medium sized relative of the mockingbird enjoys cavorting amongst dense shrubbery and generally at ground level. They make a cat-like sound, hence their name. When they build nests, it takes about a week, and they are generally built 3-5' off the ground. Their behavior when threatened is different than, say, blackbirds who enjoy dive-bombing anyone who gets near their nests. Gray Catbirds, when threatened, hop on the ground and sometimes aggressively approach the predator.

The Nest Sites
The researchers studied three nest sites in suburbs of Washington D.C. Two sites, Opal Park and Spring Park were located in Takoma, Maryland. Another site was in Bethesda, Maryland.

The Predators
Researchers fully admit there are drawbacks to how they determined the type of predators. Predators were observed during a specific time frame during the morning on roads abutting the different nest sites. Nocturnal or more hidden predators therefore would be unaccounted for. They make reasonable assumptions about the kill style of the predators.

The predators they observed include: Gray Squirrels, Chipmunks, Domestic Cats, and American Crows. Predators that would, by their behavior, be difficult to observe include: Snakes, Owls, Rats.

The researchers make presumptions about density based on previous information, as they were unable to determined species-specific predator densities at each site.

The Study Goals
Researchers asked two questions: How do the nest and post-fledging survival rate differ in a suburban environment and how do intrinsic (sex/brood size) and extrinsic (predators) affect survival probabilities.

The Results
Researchers studied 68 nest sites for 791 days. Intrinsic factors did not affect survivability in this particular study (but authors note it has in others).

61% of individuals died before reaching maturity. Of that 61%, 79% were killed by predators. And of that, 47% were killed by domestic cats.

There were eight directly observed kills, almost all committed by cats. Eleven were presumed to be committed by specific species as determined by how the bodies were found. Fourteen of the predator deaths could not be attributed to a particular animal. Two birds died for unknown reasons, two were killed because of weather-related factors, two died during window strikes and three were found without obvious symptoms.

Gray Squirrels were most prevalent at two of the sites as were cats. Crows, jays and raptors are common nest predators, but they were not responsible for statistically more deaths. However, researchers note that the presence of Gray Squirrels and domestic cats were correlated with lower survivability. Researchers note that cryptic (rats) or nocturnal predators could not be reasonably accounted for and may also pose statistically significant threat to Gray Catbird juvenile survivability.

The researchers believe this study suggests domestic cats have a disproportionate impact on the survivability of Gray Catbirds.

The Possible Drawbacks
What is great when you can read peer-reviewed journal articles in their entirety is that each article includes a Discussion section, which highlights some of the possible concerns or drawbacks of the study.

One drawback is the small sample size.

The authors in this piece offer two possibilities in regards to cats. One is that we assume the effects of cats on Gray Catbirds is additive in nature and thus cats pose a significant threat to the population of the birds. The other is that cats fill in the role of absent predators and perform the same function, weeding out weaker individuals who might have survived without predators. They suggest future research on that issue.

They specifically acknowledge that this data is correlative, not causative. That is a big deal in science. Correlation describes the relationship between two things. Sometimes it is coincidental, sometimes the correlation is mildly strong, and sometimes it is strong enough to suggest a causality - that is the presence of one "thing" results in the presence of another. In this scenario, it is acceptable to suggest there is a correlation between the presence of cats and a lower survivability of post-fledging Gray Catbirds. It is not acceptable to state that cats cause lower survivability. As mentioned above, cats may be filling a niche that is necessary in an ecological sense. They could also be putting negative pressure on the local Gray Catbird population. Other predators may be causing higher death rates but the scope of this study prohibited learning that.

My guess is this study might be used, like those before it, as anti-cat propaganda. It is my belief all companion cats should be indoor-only with supervised outdoor access for lots of reasons. The issue of feral cats is contentious and complex, and not one I plan on addressing here (or probably ever). But I do believe this study highlights the need to try really hard and objectively analyze the data presented. It is in the best interest of the non humans to do so.