Nest-attenders in the pied flycatcher (Ficedula hypoleuca) during nestling rearing: A possible case of prospective resource exploration

Visits to nest holes by birds other than their owners is a familiar phenomenon for students of breeding biology. In this study, we evaluate that behavior using a transponder reading system. Eighty-five males and females were Fitted with transponders at the end of the incubation period or just after hatching. Nest boxes were fitted with transponder readers from just after hatching until all nestlings fledged. That system revealed 123 visits by birds to nest boxes other than their own, a visit being defined as at least one visit to a separate nest box on a separate day. Males were more often detected at other nests than females (53% of males vs. 29% of females visited) and males on average made more visits than females did (4.8 vs. 2.5 visits). However, both males and females devoted time to visiting other nests while still feeding nestlings, That behavior is more common than previously suspected and is consistent with birds prospecting for future nest sites or investigating patch reproductive success.