title: Why Do Migratory Birds Sing on Their Tropical Wintering Grounds?
abstract: Many long-distance migratory birds sing extensively on their tropical African wintering grounds, but the function of this costly behavior 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 nonbreeding territories, (ii) functions as practice to improve complex songs for subsequent breeding, or (iii) is a nonadaptive consequence of elevated testosterone carryover. We found support for neither the long-assumed territory-defense hypothesis (great reed warblers had widely overlapping home ranges and showed no conspecific aggression) nor the testosterone-carryover 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 song type rather than a 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.
authors.forename: Marjorie C
authors.initials: MC
authors.lastname: Sorensen
authors.forename: Susanne
authors.initials: S
authors.lastname: Jenni-Eiermann
authors.forename: Claire N
authors.initials: CN
authors.lastname: Spottiswoode
mesh_terms: D000161:Acoustic Stimulation / Q000662:veterinary; D000818:Animals; D012621:Seasons; D020308:Songbirds / Q000502:physiology*; D013686:Telemetry / Q000662:veterinary; D013731:Territoriality; D013739:Testosterone / Q000097:blood*; D014828:Vocalization, Animal; D015024:Zambia
chemical_list: D013739:Testosterone
keywords: Palearctic-African migrants; great reed warbler; nonbreeding; song function; winter