Laura Erickson's For the Birds

Wednesday, March 26, 2008

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!

4 comments :

  1. Thank you for your article. I'm so discouraged. Today I took down all my feeders. Every time I look out my window all I see is a cat scaring the birds off or walking away with a bird in it's mouth. I feel like I'm partnering with this cute little murderous fur balls. There is no question they are devastating the bird population while theirs just goes up and up. Today I saw one of them go after a squirrel and almost get it, when we we get help?

    I think the way to handle this problem is the Indoor Cat policy. Keep cats indoors, or in large runs attached to houses. Cats are safer and healthier and so is the wildlife. Cats are NOT a part of nature. De-clawed and belled cats are still very able to catch birds. Roaming cats die much younger than cared for loved cats, kept close to home. So many cats are killed by cars.

    I don't let my dogs run through my neighbors yard or defecate in their yards, yet all the neighborhood cats wander all over yards and use the world is their toilet.

    Cat owners seem only to want pets when is it convenient for them, it is time they took responsibility for what they adopted.

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  2. I posted a link to this blog in my forum under wildlife and Feline Specific. I think this is an excellent piece of writing. ~ I also believe that people letting their cats outside contributes to a lot of problems including the killing of our birds. In these days, I think it is not the thing to do and cats should be kept inside, or on a leash and cat halter for fresh air or in a screened in porch.

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  3. In my county, if you have a cat on your property, you call animal control, they come out, set a trap, check it daily, and if they catch one it goes to the shelter. The owner can claim it, or it goes out for adoption or euthanasia. THis is the way it should be in every county. I have a friend who didn't realize he had cats visiting his yard until he found feces in his kids sandbox. He ended up trapping 15 cats in a month or so. You cannot feed birds unless you trap cats. If you do you are creating a attractive nuisance and luring birds to their deaths. It's just that simple. Some birders shoot cats in their yards, some trap. Cats are NON-native predators, north american birds did not evolve with them, and cannot cope with their predation...cats kill regardless of hunger...something that raptors and coyotes just don't do.

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  4. That's a superb article, Laura! Have you ever heard the three most important things about real estate?
    1. Location
    2. Location
    3. Location
    In my opinion, the only way we can make headway against the cat enablers is to do these three things:
    1. Organize!
    2. Organize!
    3. Organize!

    ReplyDelete