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Inbreeding in the American Crow

By Andrea Townsend
American Crow
American Crow by Paul Powers.

A little too much cooperation?

A number of scientists at the Cornell Lab of Ornithology study cooperatively breeding birds, ranging from species that breed in monogamous pairs with older offspring serving as “helpers” to species that share a single nest among many unrelated pairs or individuals. In most cases, cooperative breeders have mechanisms to help them avoid inbreeding; incest avoidance is considered one of the few near-universals in the animal kingdom. A surprising exception is the American Crow.

At left is “TU,” a male hatched in 2006. In the middle is his father. At right is a sibling from 2009. TU fed his sibling at the nest and after it fledged. Photo by Kevin McGowan.

A cooperatively breeding group of crows typically includes a breeding pair (“mom” and “dad”) and their yearling and adult offspring from previous broods. The population around Ithaca, New York, has been studied extensively by Kevin McGowan since 1988; in 2001 Anne Clark of Binghamton University began collaborating with Kevin, and several Binghamton and Cornell students have been studying this population with them. Kevin and his team have learned that Ithaca crow families may include stepparents and their stepchildren, nephews, brothers, and other relatives of the “dad,” and sometimes additional unrelated birds. These “auxiliaries” usually help in feeding the nestlings and defending the territory against predators and other crows.

For my own doctoral research, I analyzed genetic paternity using blood and feather samples collected from hundreds of nestlings, breeders, and auxiliaries. This research established that auxiliary males sometimes mate with “mom” (usually “stepmom”) and sire some offspring in “dad’s” nest. In some cases, the sires are in fact adult sons of “mom!”

The consequences of inbreeding appear to be grim; our research shows that inbred offspring have a lower probability of survival and are more likely to die with signs of disease. Why do crows inbreed? Is inbreeding rarer in other crow populations? Might there be benefits to inbreeding? Continuing long-term study into this population may uncover some interesting answers.


Originally published in the January 2010 issue of BirdScope.

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