SEXUAL DIFFERENCES IN CONSPECIFIC TERRITORIAL DEFENSE OF MARSH HARRIERS (CIRCUS-AERUGINOSUS)

Observations of six pairs of marsh harriers (Circus aeruginosus) in th e Upper Ebro Valley (Navarra, Spain) during the incubation and nestlin g periods showed that: (1) males defended their territories against co nspecific intruders more often than females, (2) responses were more f requent to intrascxual as opposed to intersexual intrusions, and (3) f requency of responses between opposite sexes increased when the breedi ng season progressed. The greater rate of defensive responses of males toward other males and tolerance of females by males at the onset of reproduction supported the idea that conspecific territorial defense d uring the breeding season functions to protect the nesting area and th e pair bond. Alternatively, sex difference may reflect differences in the costs of defense in relation with aerial agility of the sex.