Fourth International Behavioral Ecology Congress, Princeton, NJ, August 1992.
Comparative Social Biology of Two Sympatric Crows. Kevin J. McGowan. Section of Ecology and Systematics, Cornell University, Ithaca, NY 14853, USA.
American Crows (Corvus brachyrhynchos) and Fish Crows (C. ossifragus) are sympatric in parts of their ranges. American Crows are known to have helpers-at-the-nest throughout much of their range, while helpers are rare in Fish Crows. I examined reproductive biology and seasonal changes in social behavior in both species in central New York, and Fish Crows in Florida, in an attempt to determine the factors supporting cooperative breeding in one species but not the other. Auxiliaries, primarily non-breeding young from previous years helping parents, were present at 46 of 65 American Crow nests, but none were observed at 21 Fish Crow nests. Both species are typical crows in that both are large, long-lived birds that eat a variety of foods and delay reproduction until they are several years old. Reproductive success was similar for the two species, and both had similar constraints on their breeding. Starvation appeared to be more important than predation in limiting fledging success. The most important difference noted was that, although both species congregate in large flocks outside of the breeding season for foraging and roosting, breeding American Crows maintained territories throughout the year but Fish Crows defended only small areas around their nests during breeding.