Journal of Raptor Research, vol. 28, issue 1 (1994) pp. 23-26
Observations of six pairs of marsh harriers (Circus aeruginosus) in the Upper Ebro Valley (Navarra, Spain) during the incubation and nestling periods showed that: (1) males defended their territories against conspecific intruders more often than females, (2) responses were more frequent to intrascxual as opposed to intersexual intrusions, and (3) frequency of responses between opposite sexes increased when the breeding 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 during the breeding season functions to protect the nesting area and the pair bond. Alternatively, sex difference may reflect differences in the costs of defense in relation with aerial agility of the sex.
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