The Function and Evolution of Wood-Warbler Flight Calls (Parulidae)

Animals produce vocalizations to communicate with territorial rivals, to communicate with potential mates, to alert others of predators, or to stay in contact with other animals. To date, little research has focused on the vocalizations that animals use to communicate during migration. Many birds produce flight calls, which are short calls given by migratory birds on the wing. The function of flight calls is poorly understood but they are thought to help migrants maintain contact with other individuals during movements between breeding grounds and wintering grounds. Across four data chapters in my dissertation, I investigated the function and evolution of flight calls in wood warblers (family: Parulidae), a biodiverse group of songbirds that migrate between the Neotropics and temperate North American breeding grounds. In the first data chapter, I developed a novel three dimensional microphone array to record the flight calls of migratory songbirds on the wing. I found that warblers in flight can be triangulated with high accuracy in three dimensions using a microphone array. In the second data chapter, I test the hypothesis that warblers with similar calls migrate in close proximity. I triangulated flight calls of 18 species of wood warblers using a microphone array. I found that triangulated warblers migrate in closer spatial proximity to acoustically similar flockmates. In the third data chapter, I test the hypothesis that warblers with similar migrations have similar flight calls. I quantified patterns of acoustic variation using a large library of call recordings, and I quantified the migration patterns of each warbler species using fine-scale spatial data. I demonstrate that warblers with similar breeding latitudes, wintering ranges, and timings of vernal migrations have similar flight calls. In my fourth and final data chapter, I examined the evolution of warbler flight calls, by performing ancestral state reconstructions of flight call evolution and examining if warblers with similar flight calls had evolved similarity following a model of convergent evolution. I found that 12 species of warblers with highly similar calls (the “Zeep complex”) have evolved similar calls following a model of convergent evolution. Taken together, these results suggest that flight calls have evolved to be similar among species that share similar migrations, that unrelated species have evolved acoustically convergent calls when they share similar migrations, and that warblers appear to congregate with similar sounding flockmates. Together, these results suggest that warbler flight calls are important for maintaining migratory flocks, and function as in-flight indicators that warblers use to guide their migratory behaviour