Data from: The cost of prospecting for dispersal opportunities in a social bird

Understanding why individuals delay dispersal and become subordinates within a group is central to studying the evolution of sociality. Hypotheses predict that dispersal decisions are influenced by costs of extra-territorial prospecting often required to find a breeding vacancy. However, little is known about such costs, partly because it is complicated to demonstrate them empirically. For example, prospecting individuals may be of inferior quality already before prospecting and/or have been evicted. Moreover, costs of prospecting are mainly studied in species where prospectors suffer from predation risk, so how costly prospecting is when predators are absent remains unclear. Here, we determine a cost of prospecting undertaken by subordinate Seychelles warblers, Acrocephalus sechellensis, in a population where predators are absent and individuals return to their resident territory after prospecting. Prospecting individuals had 5.2% lower body mass than non-prospecting individuals, and our evidence suggests this may be due to frequent attacks by resident conspecifics, which likely leads to reduced food intake. These results support the hypothesis that energetic costs associated with dispersal opportunities is one factor influencing dispersal decisions and shaping the evolution of delayed dispersal in social animals.