Extra-Pair Courtship and Copulation Attempts in Northern Orioles

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Abstract
I observed 50 cases of male Northern Orioles (Icterus galbula) intruding on other males'' territories. In 23 of 50 cases, the intrusion led to an extra-pair courtship or chase. Three of these intrusions resulted in extra-pair copulation. Intrusions were significantly more likely during the female''s fertilizable period (during egg laying and up to 5 days before the first egg was laid). Intrusions occurred at the same time in the nesting cycle as pair copulations and both peaked during early morning hours. Both yearling and adult males intruded, but only adult males engaged in extra-pair copulation. In all cases where the intruding male was identified by color bands, he was a paired male from a territory 110 m to 280 m away from the target female''s nest. Five unbanded intruders came from more than 200 m away. All but one female ignored or flew away from males performing extrapair courtship. Most females gave loud calls when fleeing from intruders attempting extrapair copulation; in two cases females were driven to the ground and mounted. Paired males showed three types of mate association when females were fertilizable: (1) "consorting," accompanying their mate when they left the territory to forage or gather nesting materials; (2) "driving," when females moved close to neighboring males and were chased back into the territory; and (3) rapid pair copulation. The last behavior was observed only during song playback experiments that may have been perceived by resident males as territorial intrusions.