The dimensions of the cat situation

Rob Fergus made the following interesting calculations on a conservation listserv last week, and kindly gave me permission to quote:

The problem is there are no agreed upon numbers of birds killed by cats each year. The trick is to come up with a calculation based on:

a) The numbers of cats roaming the landscape
b) The number of birds killed by the average cat

There is no agreement about either of these two figures, so the trick is to try and come up with a number that you can most solidly defend.

Here's my take on it:

Number of cats
Really there are three important numbers here, the number of pet cats, the number of those cats that are allowed outside, and the number of feral or stray cats. The first figure is the easiest to come close to. The American Veterinary Medical Association provides numbers of pet cats (and other animals)

They calculate that in 2007 there were 81,721,000 pet cats in the U.S.
(http://www.avma.org/reference/marketstats/ownership.asp).

Now you have to determine how many of those cats are allowed to roam outside and potentially kill birds. According to the $1,195 American Pet Products Manufacturers Association survey (http://www.appma.org/pubs_survey.asp), 43% of cat owners allow their pets to roam outside (as quoted by the Cat Fanciers Association: http://www.cfainc.org/articles/legislative/confinement-laws.html).

If we accept these numbers (and they are probably the least controversial of all the numbers here), that gives us:

35.1 million outdoor pet cats in the U.S.

Now we have to add the number of feral and stray cats. This number is a lot squishier. We need better numbers here for sure. I haven't seen a good study on this, but the numbers published by feral cat advocacy groups seem to range between 60-100 million cats. In the absence of good numbers, for now you can probably get away with saying there are as many feral and stray cats as there are owned cats. So lets say 81 million again.

So that's 81.7 million + 35.1 million = 116.8 million outdoor cats

More realistic might be a range of 95.1 to 135.1 million (based on possible feral range).

But for arguments sake, lets just stick with 116.8 million cats for now.

How many birds killed by cats?
Here's where the cat advocates want to really fight about the numbers. But here are some options--

According to a study in Michigan by Lepczyk et al (online at http://www.csis.msu.edu/Publication%20files/Landowners%20and%20cat%20predation%20across%20rural-to-urban%20landscapes.pdf)
Outdoor pet cats across an urban to rural gradient killed an average of .683 birds each week during the breeding season.

IF you can extrapolate that across the full year, that would be an average of 35.5 birds killed by each cat/each year. IF you can use that figure for all outdoor cats, you get a calculation of 4.1 billion birds killed each year.

But maybe cats don't kill birds at the same rate all year long, or at the same rate everywhere that they do in Michigan. But lets presume that the ONLY kill birds during the breeding season (22 weeks in MI), that would still be 1.76 billion birds killed per year.

Another study in San Diego (Crooks and Soule 1999 cited here: http://www.njaudubon.org/Conservation/CatsIndoors/cats/catpreNJAS.pdf) found each cat to kill an average of 15 birds per year (and 41 other small animals). IF you multiply this number by the number of outdoor cats you get 1.75 billion birds killed per year. And that's just in the U.S. and doesn't take into account our migratory birds killed by cats in Canada or Latin America.

You can play this game all day, based on numbers from studies. The cat advocates will try to cast doubt on these predation rates, but there are arguments to be made that real average predation rates may be higher (these are mostly studies of owned cats which may hunt less, owners may not be seeing all birds killed by their cats and consumed or left elsewhere, etc.).

So what's the number? I think you could make a strong argument for the 1.7 billion based on either the San Diego study or the MI study. If you wanted to be more conservative, you could probably say "at least 1 billion birds a year and quite probably higher". That's what I've said at Audubon. That would still be an order of magnitude higher than the cat people you encounter will want to accept. You can read their own thoughts about this in a series of articles here: http://www.straypetadvocacy.org/html/cat_predation.html

It's tricky, but I think the low numbers promoted by the cat advocates contain many more flawed assumptions than these estimates here, and are not based on the "best available science".

Hope this helps outline some of the issues involved with coming up with a number!