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 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.许多长途迁徙鸟类会在其非洲热带越冬地进行大量鸣唱，但这类高能耗行为的功能至今仍未明确。本研究首次针对三项互斥假说开展实证检验：我们结合了对非洲越冬的大苇莺（Great Reed Warbler，*Acrocephalus arundinaceus*）的野外研究，以及跨古北界-非洲区迁徙鸣禽类群的比较分析。我们旨在解答越冬鸣唱的三项潜在功能：(i) 用于防御非繁殖期领地；(ii) 作为练习以优化后续繁殖所需的复杂鸣唱；或(iii) 是睾酮残留水平升高带来的非适应性副产物。研究结果既未支持长期以来被认可的领地防御假说（大苇莺的家域存在广泛重叠，且未表现出同种攻击性），也未支持睾酮残留假说（大苇莺的越冬鸣唱与血浆睾酮浓度无关联）。与之相反，我们获得了对鸣唱优化假说的最强支持：大苇莺在非洲越冬时鸣唱的是求偶鸣唱类型，而非领地鸣唱；且在非洲越冬期鸣唱最频繁的类群，正是那些性选择对鸣唱特征作用最强的类群——它们拥有更复杂的鸣唱结构，且更大概率呈现性单态性（雌雄外观无显著差异）。本研究凸显了性选择如何在全年生活周期中对动物生态学特征产生深远影响。