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Based on comments by some readers about how they have appreciated the discussion about cats/birds, here are some additional thoughts sent to Jane offline in answer to her request for studies that have reported bird declines due to cat predation.
 
Here are a couple of studies:
 
Lepczyk, et. al. 2003. Landowners and cat predation across rutal-to-urban landscapes. Biological Conservation. 191-201.
    This report looked at cat predation along Breeding Bird Survey routes in Michigan; 23+ species of birds documented killed by cats, based on homeowner surveys, including Eastern bluebird and hummingbird, two species considered to be of conservation concern.  The authors concluded that cat predation plays an important role in fluctuations of bird populations and should receive more attention.
 
Balogh and Marra. 2008. Of cats and catbirds: juvenile post-fledgling survival of Gray Catbirds in a suburban world.  Presented at the 126th meeting of the AOU.  Study in suburban Washington DC where 47% of predation mortality of catbird fledglings was attributed to cats.
 
There are several others cited in an American Bird Conservancy fact sheet that is available at abcbirds.org
 
Answering the question about bird declines due to cat predation is not easy because it depends on your scope or scale of concern.  If the question is, are cats are causing the decline of a species, the answer is probably yes.  Some individual populations of some birds, gray catbird for example (based on the paper cited above), are turning into sinks or disappearing altogether, partially as the result of cat predation.  As the number of populations lost increases we would have to logically conclude that the species is suffering as a whole, and that a large portion of that trauma can be attributed to cat predation.  The lesson here is that it's impossible to assign blame in regards to the decline of a species.  Species decline, become rare, endangered, and eventually extinct for a wide variety of reasons, often because of multiple causes.  As we've seen mentioned in other articles, human-caused mortality of birds is not just by cats, but even
 greater numbers are attributed to collisions with buildings, towers, and other tall objects, automobile roadkills, and pesticides.  Reportedly 10, 15, maybe 20% of the North American avifauna is lost annually by these several human causes, and that doesn't even include habitat destruction, an indirect mortality factor.  Unfortunately there is little we can do about most of these things.  We're not going to tear down skyscrapers and towers because they stick out along traditional bird migraory routes.  In fact, we're putting up more obstructions in the form of microwave towers and wind turbines.  We're not going to win the battle against pesticides and other toxic chemicals any time soon, and we certainly have little to say about pesticide use in South America.  At least controlling our cats is something an individual can do, and now we're getting a better handle on how much of an impact that might actually make if we could convince enough people
 to do so.  I know, it will be like trying to get everyone to install solar panels.
 
Rather than having to worry about the eventual decline of a species I prefer to focus on the local community, and the local biodiversity that sustains the ecosystem around which we build our local communities.  You (as a hypothetical person) may live in a community without any cats, in which case your responsibility in preserving your local biodiversity should be translated to some other bad habit, like reducing your lawn, or planting native shrubs in replacement of useless ornamentals.  But apparently, most communities do not have this luxury.  Although they should also be using more biotically friendly practices in their backyards, they have the added responsibility of controlling their cats.  Maybe in doing so they will eventually only save the robins or chickadees or others we consider to be common, but the loss of even one species from a local community is ecologically significant.
 
Rick Enser, Braintree