Extraterritorial visits in a cooperatively breeding songbird are consistent with multiple functions

Cryptic interactions among individuals in animal societies can be challenging to detect due to their rarity or their covert nature, but they can have important fitness consequences for the individuals involved. Using radiofrequency identification (RFID) technology, we examined the pattern and function of visits to the nests of conspecifics in western bluebirds, Sialia mexicana. Western bluebirds live in kin neighbourhoods year-round and exhibit a low frequency of cooperative breeding. Various functions have been proposed for extraterritorial visits, but few studies explore multiple potential functions. We asked whether extraterritorial visits in western bluebirds are consistent with (1) seeking extrapair copulations (EPCs), (2) prospecting for future nest sites or mates and/or (3) maintaining relationships with kin. We found some evidence supporting all three functions of visits. Postdawn visits made by males to unrelated females were significantly more likely to occur during the period in which males seek within-pair matings than outside this period. We found that visiting a given nestbox or a given potential mate was associated with an increased probability of nesting in that box or with the potential mate the subsequent year, suggesting that some visits are linked to prospecting. Finally, males preferentially visited kin: the proportion of visits to relatives was greater than the expected mean proportion of visits if males visited boxes randomly while travelling the same distance. Our work supports multiple functions of extraterritorial visits and highlights the importance of rare and cryptic behaviours to understanding social behaviour.