Extra-pair mating effort of male hooded warblers, Wilsonia citrina.

Few researchers have been able to quantify the time or effort expended by male birds in seeking extra-pair copulations with neighbouring females. In this study, we radio-tracked male hooded warblers, to determine the frequency and duration of intrusions onto neighbouring territories. Extra-pair fertilizations are common in hooded warblers, with 35% of females producing extra-pair young. Males intruded onto territories where females were nest building more often than expected by chance and approached females during most intrusions (78%), suggesting that these intrusions were extra-pair copulation attempts. Almost all intrusions (96%) were to adjacent territories, and males made an average+/-se of 0.4+/-0.2 forays/h and spent 4.7+/-2.6% of their time off-territory. Males were involved in fights or chases with the resident male in 21% of intrusions, but were apparently undetected during most intrusions. Males with high intrusion effort onto neighbouring territories had the lowest rate of intrusions onto their own territory by other radio-tracked males. Males varied extensively in their intrusion effort, but this was not significantly correlated with male body mass, age or song rate. The percentage of time spent intruding onto other territories was small (0-8%), so males probably do not face strong trade-offs between making extra-pair copulation forays and other activities like mate guarding and feeding offspring. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.