Male extraterritorial behavior predicts extrapair paternity pattern in blue tits, Cyanistes caeruleus

In territorial species, at least 1 of the 2 individuals involved in an extrapair mating has to leave its territory in order to copulate with the extrapair partner. However, data on extraterritorial forays are difficult and time-consuming to acquire, and extensive datasets on foraying behavior are rare. Here, we use an automated recording system to investigate forays of individual blue tits (Cyanistes caeruleus) to other pair’s nest-boxes during early spring, when extrapair matings occur. The study has 3 aims. 1) To describe daily and seasonal variation in the frequency of foraying behavior. 2) To investigate which individuals perform forays. 3) To investigate whether male and female visits to other pair’s nest-boxes predict the occurrence of extrapair paternity. We find that males visited foreign nest-boxes more frequently than females, and that the majority of visits occurred in the morning hours, but not around dawn. Male visits to a foreign nest-box were unrelated to the social and the visited females’ fertile period. Male body size, but not age predicted the occurrence of forays. Males that forayed were more likely to sire extrapair offspring with the female they visited compared with nonvisiting males. Whether males copulate during such visits or whether females seek extrapair copulations from visiting males remains unknown. However, male forays to a foreign nest-box coincided with the presence of the resident female at the same nest-box more often than expected by chance. This indicates that visits to foreign nest-boxes could be an opportunity for extrapair mate assessment or extrapair copulations.