Nocturnal migratory departure timing of songbirds in relation to social and environmental conditions

Background/Question/Methods

: For nocturnally migrating songbirds, evening departure time limits the maximum time available for flight with implications for the overall pace of migration. Evening departure time can be influenced by extrinsic (wind, weather) and intrinsic (fuel load) conditions, but little is understood regarding the role of social information encountered by songbirds prior to departure. During migratory flight, songbirds often associate in loose flocks, and will utter distinct night flight calls (NFCs). These calls may serve to aid in flock cohesion, and behavioural responses elicited in captive songbirds suggest a possible connection with departure behaviour. We used a novel combination of marine radar, acoustic monitoring, and automated radio telemetry data collected over two years at stopover sites in New Brunswick, Canada, to evaluate how the likelihood of migratory departure was affected by social information and meteorological conditions in four songbird species. We applied Cox semi-parametric hazard analysis to evaluate departure likelihood across 20-minute intervals within departure nights, including migrant passage (measured using radar), NFCs (measured using audio recorders), and tailwind assistance (from weather data) as time-varying covariates between intervals. We expected greater departure likelihood during periods with higher migrant passage and total NFCs, and with wind conditions favorable to migratory flight.

Results/Conclusions

: Preliminary analysis of data collected from 22 Swainson’s Thrush (Catharus ustulatus) and 63 White-throated Sparrow (Zonotrichia albicollis) found greater departure likelihood (earlier departure) in Swainson’s Thrush, and an increased departure likelihood with greater tailwind assistance in Swainson’s Thrush. Neither migrant passage nor age class had any significant effect on departure likelihood in either species. Preparation of additional automated telemetry, radar, and acoustic data is ongoing, and these were not included in this preliminary analysis. Early results suggest that environmental factors and intrinsic species differences predominantly influence nocturnal departure timing, rather than locally acquired social information.