Male Acadian flycatchers, Empidonax virescens, obtain extrapair fertilizations with distant females

We used genetic parentage analysis and radiotracking to determine how far male Acadian flycatchers travel from their territory in search of extrapair matings. Acadian flycatchers on our study site have a linear spatial distribution of territories associated with watercourses. With this type of spatial arrangement, males have only one or two immediate neighbouring females within 100 m and therefore have limited opportunities for extrapair (EP) fertilizations unless they travel far from their territory. In our sample of 53 nests collected over 4 years, 58% of nests contained EP young and 40% of nestlings were EP young. We identified the EP sire and the location of his territory at 13 nests with EP young, and over half (7/13) were not immediate neighbours. The mean distance between the territory of the EP sire and the territory where he attained EP fertilizations was 526 m. We were also able to exclude all immediate neighbours as EP sires for 12 additional nests containing EP young. The minimum average distance between an EP sire's territory and the territory containing an EP young that he sired was estimated to be 910 m. Overall, the EP sire was a neighbour in only 24% of nests with EP young. Radiotelemetry observations showed that six of seven mated males made off-territory forays and averaged 0.4 forays/h. Males travelled an average of 202 m off-territory (range 50-1500 m) and were away from their territories an average of 10.2 min/foray. Because our findings contrast sharply with those of other studies examining extrapair fertilization in passerines, in which most extrapair sires are neighbours, we suggest that when males have few adjacent neighbouring females, the benefits of searching widely to increase the number of potential extrapair mates can outweigh the costs of EP forays.