Data from: Why do migratory birds sing on their tropical wintering grounds?

Many long-distance migratory birds sing extensively on their tropical African wintering grounds, but the function of this costly behaviour remains unknown. In this study we carry out a first empirical test of three competing hypotheses, combining a field study of great reed warblers (Acrocephalus arundinaceus) wintering in Africa with a comparative analysis across Palearctic-African migratory songbird species. We asked whether winter song (i) functions to defend non-breeding territories, (ii) functions as practice to improve complex songs for subsequent breeding, or (iii) is a non-adaptive consequence of elevated testosterone carry-over. We found support for neither the long-assumed territory defence hypothesis (great reed warblers had widely overlapping home ranges and showed no conspecific aggression), nor the testosterone carry-over hypothesis (winter singing in great reed warblers was unrelated to plasma testosterone concentration). Instead, we found strongest support for the song improvement hypothesis, since great reed warblers sang a mate attraction rather than territorial song type in Africa, and species that sing most intensely in Africa were those in which sexual selection acts most strongly on song characteristics: they had more complex songs and were more likely to be sexually monochromatic. This study underlines how sexual selection can have far-reaching effects on animal ecology throughout the annual cycle.