Bird-killing cats may be one of nature’s ways of making better birds

Death is not pretty, early in the morning on the doorstep. A small dead bird was left at our front door one morning last week, lying by the newspaper as if it might at any moment fly away. I knew it would not. Like other birds before it, it was a gift to our household by Feisty, a cat who lives with us. Feisty is a killer of birds, and every so often he leaves one for us, like rent.

We have four cats, and the other three, true housecats, would not know what to do with a bird. Feisty is different, a long-haired grey Persian with the soul of a hunter. While the other three cats sleep safely in the house with us, Feisty spends most nights outside, prowling.

Feisty’s nocturnal donations are not well received by my family. More than once it has been suggested, as we donate the bird to the trashman, that perhaps Feisty would be happier living in the country.

As a biologist I try to take a more scientific view. I tell my girls that getting rid of Feisty is unwarranted, because hunting cats like Feisty actually help birds, in a Darwinian sort of way. Like an evolutionary quality control check, I explain, predators insure that only the more-fit individuals of a population contribute to the next generation, by the simple expedient of removing the less-fit. By taking the birds who are least able to escape — the sick and the old — Feisty culls the local bird population of less fit individuals, leaving it on average a little better off.

That’s what I tell my girls. It all makes sense, from a biological point of view, and it is a story they have heard before, in movies like Never Cry Wolf, and The Lion King. So Feisty is given a reprieve, and survives to hunt another night.

What I haven’t told my girls is how little evidence actually backs up this pretty defense of Feisty’s behavior. My explanation may be couched in scientific language, but without proof this “predator-as-purifier” tale is no more than a hypothesis. It might be true, and then again it might not. By such thin string has Feisty’s future with our family hung.

This week the string became a strong cable. Two French biologists put the hypothesis I had been using to defend Feisty to the test. To my great relief it proved true.

Drs. Anders Moller and Johannes Erritzoe of the Universite Cure in Paris devised a simple way to test the hypothesis. They compared the health of birds killed by domestic cats like Feisty with that of birds killed in accidents such as flying into glass windows or moving cars. Glass windows do not select for the weak or infirm — a sickly bird flies into a glass window and breaks its neck just as easily as a healthy bird. If cats are actually selecting the less-healthy birds, then their prey should include a larger proportion of sickly individuals than those felled by flying into glass windows. How can we know what birds are sickly? Drs. Moller and Erritzoe examined the size of the dead bird’s spleens. The size of its spleen is a good indicator of how healthy a bird is. Birds experiencing a lot of infections, or harboring a lot of parasites, have smaller spleens than healthy birds.

They examined 18 species of birds, more than 500 individuals. In all but two species (robins and goldcrests) they found that the spleens of birds killed by cats were significantly smaller than those killed accidently. We’re not splitting hairs here, talking about some minor statistical difference. Spleens were on average a third smaller in cat-killed birds. In five bird species (blackcaps, house sparrows, lesser whitethroats, skylarks, and spotted flycatchers) the spleens of birds pounced on by cats were less than half the size of those killed by flying at speed into glass windows or moving cars.

As a control to be sure that additional factors were not operating, the Paris biologists checked for other differences between birds killed by cats and birds killed accidentally. Weight, sex, and wing length, all of which you could imagine might be important, proved not to be. Cat-killed birds had, on average, the same weight, proportion of females, and wing length, as accident-killed birds.

One other factor did make a difference: Age. About 50% of the birds killed accidently were young, while fully 70% of the birds killed by cats were. Apparently its not quite so easy to catch an experienced old codger as it is a callow youth.

So Feisty goes on, doing Darwin’s duty. My girls have been informed that the birds he catches would soon have died anyway. But a dead bird on a doorstep argues louder than any science, and they remain unconvinced.

However, yesterday morning I came into the kitchen to find my wife cheering and pointing excitedly out the window. In the backyard a mockingbird was zooming down on Feisty, diving and pecking, driving him across the yard. Not all birds in our yard are sickly, it appears. Feisty had better hope the neighborhood birds haven’t cut a deal with Darwin, and evolved a cat-attacking ability in response to his predation. In predator-prey interactions, evolution can cut both ways.

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