PS 84-241
A competition experiment between wintering redstarts and resident Yellow Warblers in Jamaican mangrove forest
Every autumn, billions of migratory songbirds depart North America for the Neotropics, where they spend ~7 months wintering alongside ecologically similar year-round residents. In the Caribbean, the winter influx of migrants coincides with low arthropod abundances, and food shortages limit wintering songbird populations. Despite recent progress, the question remains: how do so many ecologically similar migrants and residents coexist during the most resource-poor time of year? Food-based competition likely allows coexistence of migrant and resident birds, yet no published study in any system has experimentally demonstrated that interspecific competition for food exists between migrants and residents. The primary objective of this study was to determine whether interspecific competition exists between resident Yellow Warblers (Setophaga petechia; hereafter “yellows”) and wintering American Redstarts (Setophaga ruticilla), a critical step toward understanding the importance of food-based competition as a mechanism influencing avian population processes. To test the interspecific competition hypothesis, we performed two experiments in Jamaican mangrove forests: 1) a removal experiment in which we 3D-mapped yellow territories (n = 14) and overlapping redstart territories (n = 24) before and after removal of yellows, and 2) a natural experiment in which we compared redstart presence with and without territorial yellows (n = 10 territories).
Results/Conclusions
In the removal experiment, we found no evidence that redstarts expanded their 3D territories (i.e. spatial niches) into those of removed yellows; however, due to a large population of floaters, yellow territories could only be kept vacant for 24-hrs, so redstarts may not have had time to respond. In the natural experiment, when the yellows were on the opposite sides of their territories from redstarts (i.e. absent) redstarts were 2.3 times more likely to be present than when yellows were present, indicating that yellows exclude redstarts from space, but that the space they are excluded from varies spatiotemporally depending on where the yellows are located within their territories. The relentless aggression of larger yellows towards smaller redstarts almost certainly denies redstarts access to otherwise obtainable resources: space and insect prey. This resource exclusion likely has population-level consequences over an ~7 month wintering season; thus we predict redstarts wintering in mangroves without yellows will show: 1) increased density, 2) improved body condition and 3) earlier departure date. Only by understanding interspecific competition between migrant and resident birds can we quantify the challenges that migrants face throughout the annual cycle.