Who initiates extrapair mating in song sparrows?

Extrapair paternity (EPP) is a common feature of many mating systems. Although molecular methods have made it possible to document the rate of EPP across numerous taxa, we still lack an understanding of how and why EPP happens. Behavioral data on mating interactions are needed to answer this question. We employed radiotelemetry to follow the movement patterns of tagged song sparrows (Melospiza melodia) during their nesting cycle. We found that females and males commonly forayed onto neighboring territories in the prefertile period, frequently together. In the fertile period, however, the foray rate dropped significantly, and females largely stayed on their own territories. Concurrently in the fertile period, both the time mates spent in proximity and intrusion rates on their territories increased compared with the prefertile period. After the female started laying eggs, proximity of the mates to one another declined again, and some males started to foray onto neighboring territories, in most cases into territories where there was a fertile female. Thus, females do not seem to seek out particular males for extrapair copulations (EPCs) in their fertile period, but rather it is the males who actively seek extrapair mating. Males therefore face a trade-off between ensuring paternity at home and pursuing copulations elsewhere. Agonistic interactions between extrapair males and females were almost entirely absent, suggesting that males do not force females into copulations. Therefore, in song sparrows, males have to initiate EPCs by seeking out fertile females, suggesting that males probably drive the pattern of EPP in our population.