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Cats and Predation
By: Alley Cat Rescue
A summary of an article on cats and predation by George Johnson, biology professor at Washington University.
One of Dr. Johnson's four cats, Feisty, was a hunter. Dr. Johnson's knowledge of biology told him that "predators ensure 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 that 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."
Two French biologists put his hypothesis to the test. Drs. Andre 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 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 — if cats are actually selecting the less - healthy birds, then their prey should include a larger proportion of sickly individuals that those felled by flying into glass windows.
The scientists examined the spleens of 500 birds. Sick birds harboring parasites have smaller spleens than healthy birds. They found that the spleens of birds killed by cats were significantly smaller than those killed accidentally.
Spleens were on average a third smaller in cat-killed birds. Another important fact was that 50 percent of the birds killed accidentally were young. Seventy percent of the cat-killed birds were young.
Again this proved the hypothesis that cats kill the old, sick and very young. Healthy, older birds are too experienced to be taken by cats.
This has led Dr. Johnson to state that: "bird-killing may be one of nature's ways of making better birds."
Hunting cats like Feisty actually help birds, in a Darwinian sort of way.
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