Influences of Territory Composition and Interspecific Competition on Red‐Tailed Hawk Reproductive Success

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In north—Central Oregon, Red—tailed Hawks (Buteo jamaicensis) exhibited a high degree of stability in territory occupancy and territorial boundaries over a 10—yr period. Long—term reproductive success varied markedly among territories and was correlated primarily with the dispersion and density of perches used as foraging sites and secondarily with the abundance of ground squirrels, Principally the Belding's Ground Squirrel (Spermophilus beldingi). These results imply that prey availability is more important to reproductive success than absolute prey abundance. Pairs in territories with few or poorly dispersed perches did not rely more on energetically expensive aerial foraging. However, poor dispersion of perches resulted in both decreased flight time and in decreased foraging energy expenditure. Thus it is likely that Red—tailed Hawks in territories with few or poorly dispersed foraging perches deplete the prey near the perches that are present. Territory size was not correlated with reproductive success. Red—tailed Hawks coexisted with Swainson's Hawks, and showed strong interspecific territoriality. Some pairs of Red—tailed Hawks lost portions of their territories to the later arriving Swainson's Hawks each year. Yet the contention at territorial boundaries and loss of territory to Swainson's Hawks had no observable effect upon Red—tailed Hawk reproductive success.