Hopefully my last cat and bird post for awhile!
Some folks don't seem willing to accept the magnitude of the cat predation problem. While it is a bit tricky to come up with solid numbers of birds and other animals killed, we can make estimates based on a growing number of studies of cat predation.
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 fairly defensible number.
Here's one quick look at 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.
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 43% of cat owners allow their pets to roam outside (as quoted by the Cat Fanciers Association).
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 it is probably safe to presume that 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 simplicity and for arguments sake, lets just stick with 116.8 million cats for now.
How many birds killed by cats?
Here's where it gets trickier, but here are some good options--
According to a study in Michigan by Lepczyk et al, 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, for example, if we presume that cats everywhere ONLY kill birds during a 22 week breeding season (and we know THAT isn't true!), that would still be 1.76 billion birds killed per year.
Another study in San Diego (Crooks and Soule 1999 cited here) 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 various 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? A calculation of 1.7 billion birds based on either the San Diego study or the MI study seems reasonable. A more conservative statement might be "at least 1 billion birds a year and quite probably higher". That's what I generally say. That would still be an order of magnitude higher than many people will want to accept. But it seems to be a conservative calculation. You can read what the cat advocates think in a series of articles here
From my perspective, the cat advocate positions linked here seem to make many more unwarranted assumptions than these quick calculations based on the best available science. Cat predation of birds in the U.S. seems to be on the magnitude of a billion birds a year, rather than any lower numbers reported elsewhere.