TERMINATION REPORT for U.S. Dept. Agriculture, Hatch Project NYC-183429 (revised)

REPRODUCTIVE AND SOCIAL BEHAVIOR OF TWO CROW SPECIES IN NEW YORK

PI: MCGOWAN, KEVIN J.

ECOLOGY & SYSTEMATICS, CORNELL UNIVERSITY

12 December 1997

Crows are migratory game birds and can be local crop pests. This study provided much new information about the demography and behavior of two crow species. 560 American Crow (Corvus brachyrhynchos) nests were observed in Tompkins Co., NY from 1988 through 1997, and 651 young banded and marked for individual identification. 22 Fish Crow (Corvus ossifragus) nests were studied, and 17 young marked.

The primary findings are summarized below.

• American Crows are cooperative breeders. Offspring remained with their parents for one to six years and helped their parents raise young. Family size reached 15, with young crows in five different years present at one time. 80% of observed nests had helpers. Some individuals left the parental group and helped a sibling who had a breeding spot.

• American Crows in New York hold territories throughout the year. Crows left the territory to forage and roost, especially in winter. Non-breeders left frequently, or even migrated to Pennsylvania, then returned.

• Foraging flocks occurred in areas of large food supplies, primarily outside the breeding season. All age and status classes of crows joined these flocks. Flock membership changed daily.

• American Crow nest success averaged 60%. Chief predators were Great Horned Owls (Bubo virginianus), Red-tailed Hawks (Buteo jamaicensis), and raccoons (Procyon lotor). Fledgling production averaged 1.8 per nest, and 3.2 per successful nest. Nearly half the eggs produced were lost via egg inviability and active brood reduction and removal of nestlings.

• Approximately half of fledglings survived until the next breeding season. Yearly survival was estimated at 75% for helpers and 93% for breeders.

• Young crows delayed breeding from 2 to 6 years or more. Average age of first breeding was 3.3 years for females (n=19) and 4.9 for males (n=13). Dispersal to breeding ranged from 0 to 60 km. Males were more likely to inherit all or part of their natal territories, and tended to disperse shorter distances (males = 1.1 km, n=17; females = 6.2 km, n=19).

• Crows have only recently begun nesting in cities and towns. Urban crows had nest success similar to rural crows, but produced fewer, smaller young per nest. The effect of a drought year on nestling production was more pronounced on urban crows, indicating that food supply is less dependable than in rural areas.

• Although Fish Crows are similar to American Crows in food habits and appearance, they differed in many ways.
  • Fish Crows defended small territories at the nest site only during breeding season. When not breeding, Fish Crows congregated at dependable foraging areas. Fish Crows joined American Crows in foraging flocks and nocturnal roosts.
  • Fish Crows did not have helpers attending their nests. No marked Fish Crow was ever seen attending the nest of its parents. A third Fish Crow was observed at several nests and they even fed nestlings, but in all cases the parents reacted aggressively toward the third individual. It is likely that the extra individuals were unpaired birds trying to interfere with the nesting attempt rather than a contributing helper.
  • Nest success averaged 65.%, with 2.4 young produced per successful nest. Factors affecting nest success appeared the same as in American Crows.

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