Preface This thesis is written in four parts. The first is a brief introduction, followed by two chapters formatted as journal articles, complete with their own abstracts. The final chapter presents some additional discussion material and a brief concluding paragraph. All supplementary material is presented in one appendix following the final chapter. Abstract for Chapter 2: For migrant songbirds, fine-scale movements during the post- fledging period reflect the need to optimize their occupied habitat to meet food requirements and avoid predation. Broad-scale post-fledging movements also occur but are poorly understood. They are hypothesized to aid in the development of navigational abilities, to allow individuals to prospect for future breeding territories, or to be the initiation of migration Using an automated radio telemetry array, I compared broad-scale post-fledging movements of hatch-year and adult Blackpoll Warblers (Setophaga striata) and “Myrtle” Yellow-rumped Warblers (Setophaga coronata coronata), two closely related species with contrasting life-history strategies (long-distance migrant vs. short-distance migrant) breeding in sympatry. Individuals of the two species were affixed with VHF radios on offshore islands in Nova Scotia and tracked across the entire Gulf of Maine region for up to 72 days after fledging. Departure date and five movement metrics (daily probability of initiating a flight, distance of daily displacement, total displacement, net displacement, and straightness of path) were determined and assessed within the context of two hypotheses for broad-scale movements: commencement of migration and regional exploration. Hatch-years of the two species exhibited very different behaviours. Myrtles departed their breeding grounds later, made fewer regional scale flights, and were more directional in their movements compared to blackpolls. They had a higher net and total displacement than blackpolls, even though daily flight distances were similar. Myrtle movements generally terminated along the Atlantic US coast, whereas blackpoll movements terminated throughout the region. These results are consistent with the 8 hypothesis that hatch-year blackpolls are exploring their natal region on a broad-scale and that myrtles are simply leaving the region in the initial stages of migration. Between adult and hatch-year blackpolls, there were no differences in total or net displacement, daily flight length, or the straightness of their path throughout the region, although adults did depart later and move less frequently than hatch-years. Species differences may be related to their respective migratory strategies (long-distance vs. short-distance), where the need to acquire information during post-fledging for navigational purposes is higher for blackpolls than myrtles. Age-related differences may be ascribed to experience. Abstract for Chapter 3: Nocturnal flight behaviour in migratory songbirds is influenced by a variety of intrinsic and extrinsic factors including wind conditions. Songbirds select nights to fly with supporting tailwinds for migratory flights. This selection process id further influenced by age, cloud cover, fuel load, geography, and time constraints. During the post-fledging period, the long-distance migrant Blackpoll Warbler engages in extensive (> 500 km) regional movements in a series of nocturnal flights, but what if anything influences wind selection during this period is unknown. Using automated telemetry and environmental track annotation services to give spatially and temporally accurate weather variables associated with nocturnal flights, I assessed the relationship between weather conditions and nocturnal flights during the post-fledging period. I hypothesized that adults are less time-constrained than hatch-years (because they have prior knowledge of the region, thus limiting the need for exploratory flights for learning purposes) and predicted that adults would make more efficient use of winds aloft relative to hatch-years. I hypothesized that under optimal migration theory all individuals become more energy-constrained as they approach migratory departure. I predicted that wind support would increase as the season progressed. Third, I hypothesized cloud cover would impede an individual’s ability to navigate during nocturnal flight, thus affecting wind support aloft. Adults favored more supportive wind conditions aloft than did hatch-years, and all individuals flew with more supportive wind conditions as the time of migratory departure approached. However, during September, adults favored less supportive winds (negative 44 wind support) compared to hatch-years (average wind support remained near zero) contrary to my predictions, suggesting that time-constraint is not the only factor affecting wind support. Adults, with both better orientation abilities and previous knowledge of the local landscape relative to hatch-years, may be willing (or able) to fly in less favorable tailwinds because they are more goal-orientated in their movements and with a greater ability to compensate for wind drift to maintain a desired course. There was a small trend of increased wind support with an increase in cloud cover, and on the most overcast nights (cloud cover > 75%), all flights had positive wind support. This suggests that individuals may allow for wind drift in lieu of actively navigating in overcast conditions.