Data for: Patterns of extra-territorial nestbox visits in a songbird suggest a role in extra-pair mating

Many animals make visits outside of their territory during the breeding period, but these are typically infrequent and difficult to observe. As a consequence, comprehensive data on extra-territorial movements at the population-level are scarce and the function of this behavior remains poorly understood. Using an automated nestbox visit tracking system in a wild blue tit population over six breeding seasons, we recorded all extra-territorial nestbox visits (n=22137) related to 1195 individual breeding attempts (761 unique individuals). Sixty-two percent of breeders made at least one extra-territorial visit between the onset of nest building and the day of fledging of their offspring, and individuals visited another nestbox on average on 11% of the days during this period. Visit behavior differed clearly between the sexes, with males making over three times as many extra-territorial forays as females. There was a strong overall seasonal decline in visit behavior, but this was sex dependent, with females showing a strong reduction in the number of extra-territorial visits before the onset of egg laying and males showing a strong and sudden reduction on the day their offspring hatched. The likelihood of visiting a particular nestbox declined sharply with the distance to that box, and blue tits almost exclusively visited direct neighbors. Individuals were more likely to have extra-pair offspring with an individual whose box they visited, but they were not more likely to disperse to a box they had visited. Thus, our results are inconsistent with the hypothesis that extra-territorial nestbox visits serve to inform dispersal decisions, but suggest that such visits are linked to extra-pair mating opportunities.

Methods
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