Songbirds can vary in physiological, morphological, and behavioral traits depending on their age class, which can affect their movement ecology. Our objective was to test whether migratory stopover duration and subsequent flight speeds differed between age classes in Black-throated Blue Warblers (Setophaga caerulescens). Birds were caught at Long Point Bird Observatory, Ontario, during their 2014, 2015, 2021 spring migration. They were aged and sexed based on plumage, photographed for measurements of wings (2021 only), tagged with radio transmitters, released, and tracked by the Motus Wildlife Tracking System, a coordinated array of automated radio receivers. Meteorological data was obtained from the NCEP/NCAR Reanalysis I project. Stopover duration was calculated as the day of capture until the day of the birds' first nocturnal migratory flight from Long Point. Ground speeds during nocturnal migratory flights were calculated from inter-tower movements and were decomposed into airspeeds and tailwind advantage by using the information on a bird's heading, wind speed, and wind direction. Time to event analysis was used to analyze stopover duration, and linear mixed models were used to analyze flight speeds.
Results/Conclusions:
Age classes differed in wing wear (LM: F1,32=7.11, p< 0.01) but not in wing pointedness (LM: F1,32=0.50, p=0.76). Seventy-two birds had sufficient detection data to be assigned a stopover duration (median = 4 days, range = 1–18 days). In the analysis of stopover duration, the hazard for departure depended on air temperature, year, and age. The hazard of departure for second-year (SY) birds was 37% of the hazard for after-second-year (ASY) birds, indicating longer stopovers by SY birds than ASY birds. Fifty-eight birds had 1-3 inter-tower movements during migratory flights in southern Ontario. The overall average airspeed was 8.8 ms-1, ground speed was 12.2 ms-1, and tailwind advantage was 3.4 ms-1. Airspeed and tailwind differed among years (LMM: F2,23=3.99, p=0.03; F2,23=5.27, p< 0.001) but not between age classes (LMM: F1,20=0.78, p=0.39; F1,20=1.13, p=0.30). To summarize, SY birds had some differences in wing morphology and consistently longer stopovers than ASY birds, but they had similar flight speeds. . Our results suggest that, in the Black-throated Blue Warbler, age-related differences in migration are manifest primarily at stopover.
