Intra- and Interspecific Aggression in House Finches and House Sparrows

Abstract

Observations were made on the intra- and interspecific aggressive behavior of wild house finches [Carpodacus mexicanus] and house sparrows [Passer domesticus] which utilized an outdoor feeder from October, 1970 to March, 1971. Also, interspecific groups of these species were observed in an environmental chamber from April to June, 1971. Seasonal and daily patterns of frequency of encounters in the wild indicated a direct relationship between the number of birds present and frequency of encounters. House sparrows were generally dominant to house finches, especially in cages. House finches, particularly males, tended to interact more with their own species than with sparrows. Sparrows, however, encountered finches and each other in about equal proportions. House finches were organized into rigid peck-right dominance hierarchies. House sparrows showed little tendency to form hierarchy systems of any kind. Instead, individuals'' ranks fluctuated continually with no apparent pattern. Three factors accounted for interspecific aggression in these species. Violations of individual distance resulted in agonistic encounters at commonly utilized resources. A tendency for interspecific aggression appears to have evolved in house sparrows in response to limited numbers of nest sites in a restricted habitat. The larger body size of house sparrows may give them an advantage in aggressive interactions with house finches.