Breeding site departure and post-departure movement strategy in Vireo olivaceus (Red-eyed Vireo)

Few studies have been able to assess the decision-making process behind migratory departures from the breeding site and the extent to which extrinsic cues that favor departure from fall stopover sites also apply to initial departure from breeding sites. Factors influencing departure may vary between species that first make short-distance regional movements vs. those that immediately begin long-distance migration, but for most migratory songbirds, it is unknown which post-departure movement strategy they use or why. We used an automated radio telemetry network to track adult male Vireo olivaceus (Red-eyed Vireo) as they departed their northwest Pennsylvania breeding site on fall migration to determine their post-departure movement strategy. We also assessed how local weather conditions, including surface pressure, humidity, precipitation, and wind, influenced the probability of departing the breeding site on fall migration. For the individuals for whom we could confidently ascertain departure day and time, 86% departed during the first 2 hr after sunset. Net displacement was positive, and birds headed directly south towards Florida. Birds were more likely to depart on nights with rising surface pressure and high humidity, and frequently departed in unsupportive wind conditions (headwinds). Our results suggest that V. olivaceus initiates true long-distance migratory flights after departing from its breeding sites. Birds may have departed under unsupportive wind conditions due to the scarcity of tailwinds at the breeding site. Alternatively, birds leaving their breeding areas in the fall may be less selective of optimal weather conditions for departure than birds departing stopover habitats, as they have higher fuel loads and ample time to compensate during the rest of their migratory journey.