Contrasting patterns of post-fledging movements of two sympatric warbler species with different life-history strategies

Broad-scale movements of migrant songbirds during the post-fledging period are hypothesized to aid in the development of navigational abilities, to allow individuals to prospect for future breeding territories (combined as regional exploration), or as representing the commencement of migration. Using an automated radio telemetry array, we compared broad-scale post-fledging movements of hatch-year individuals from two closely related species: blackpoll warblers Setophaga striata and myrtle warblers Setophaga coronata coronata. These two species have contrasting migratory strategies (long-distance vs short-distance), and we studied populations from two different islands in Nova Scotia that have different geographical landscape features. Locally-hatched individuals affixed with VHF radios in August were tracked throughout the Gulf of Maine region for up to 2.5 months after tagging. Departure date and direction, daily probability of initiating a flight, daily displacement, total displacement and net displacement were assessed to see if there was support for the commencement of migration or regional exploration hypotheses. We observed differences between both species and islands. Compared to blackpolls, myrtles departed later, had more variable timings and directions of departure, made fewer regional-scale flights, were more directional in their movements, and had higher net displacement. Total displacement and daily flight distances were similar between species. Variability of departure behaviour of myrtles was observed on the island farther from the mainland and both species made longer flights from that island. These results are consistent with the hypothesis that hatch-year blackpoll movements are a form of regional exploration and hatch-year myrtle movements represent the initial stages of migration. Species differences may be related to migratory strategy (long-distance vs short-distance), where the need to acquire information during post-fledging for navigational purposes is higher for blackpolls than myrtles. Island differences suggest that habitat quality and ecological barriers influence broad-scale movements, and myrtles are more facultative in their behaviour than blackpolls.