BUSCH, D.S.; MOORE, I.T.; WINGFIELD, J.C.; University of Washington, Seattle; University of Washington, Seattle; University of Washington, Seattle: Territorial aggression of a tropical passerine, Zonotrichia capensis, in response to a variety of conspecific intruders

The expression of territorial aggression by reproductive resident birds varies between the sexes and among different intruder types. Previous studies have predicted that individuals should be more aggressive towards conspecific intruders of the same sex and tolerate intruders of the opposite sex and immature individuals. In this study, we investigated the behavioral responses of an equatorial population of rufous-collared sparrows (Zonotrichia capensis) to a variety of simulated territorial intruder types: Singing Male, Silent Male, Female, and Juvenile. In this species, territories are used by the resident male and female and their young, and are also used by a population of floaters � mature individuals that do not hold territories. Resident males behaved differently toward the different intruder types in terms of song number, closest approach to the intruder, and time within close proximity of the intruder. Using a composite behavioral score, resident males responded most aggressively to Singing Male intruders. There was no significant variation in the response of resident females to the different intruder types. Neither resident males nor females behaved aggressively towards juvenile intruders. These results support the observational predictions for males, but not females, that individuals should be most aggressive towards members of the same sex, who pose the greatest threat in terms of cuckoldry and territorial takeover.