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

Visits to nest holes by birds other than their owners is a familiar phenome non for students of breeding biology. In this study, we evaluate that behav ior 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 h atching until all nestlings fledged. That system revealed 123 visits by bir ds to nest boxes other than their own, a visit being defined as at least on e visit to a separate nest box on a separate day. Males were more often det ected 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 whi le still feeding nestlings, That behavior is more common than previously su spected and is consistent with birds prospecting for future nest sites or i nvestigating patch reproductive success.