Nest intrusions in relation to breeding status in the spotless starling

Nest intruders are common in many avian species. In the spotless starling (Sturnus unicolor), a passerine closely related to the European starling (Sturnus vulgaris), it has been recently shown that visits to alien nests were in most cases related to get familiarity and personal information about prospected nest boxes and that collecting public information seems also to be involved in the intruding behaviour. In the present study we investigate whether nest intrusions are related to breeding status in both males and females. Individuals that had bred previously in the colony were detected as frequently as those without previous breeding experience intruding nests and the proportion of intruders that were owners and those that had not a nesting site during the current year was balanced. Males that were actively breeding and those that were not breeding intruded with similar frequencies in alien nests, while most females were not actively breeding when they were observed intruding a nest box. The nests more frequented by male intruders were those showing the highest female nestling feeding rates, but the frequency of female intrusions was not similarly affected by female owner feeding rates. The results suggest that some of the intrusions by males are related to nest acquisition although in general males seem to intrude to obtain other breeding resources, presumably extra-pair matings. Female intruders with a nesting territory but not currently breeding could be sampling alternative nesting sites while female floaters that intruded nests could be searching for nesting sites or attempting to lay parasitic eggs. Birds behaving as intruders at some moment of their stay in the breeding colony raised more fledglings that birds never observed performing this behaviour.