Nocturnal Provisioning by Swainson's Thrush

Abstract

Abstract We studied Swainson's Thrush (Catharus ustulatus) at two study areas of different latitudes to examine if parents provisioned their nestlings beyond daylight hours. We viewed 591 hrs of video from 37 nests. All nests were provisioned outside the dawn to dusk period but provisioning also continued through the night at the northern site. Night provisioners made an average (± SE) of 1.55 ± 0.18 fewer provisioning trips/hr during the pre-dawn to late evening periods compared to non-night provisioners. Night provisioners compensated with two additional trips/hr during the post-dusk period and 2.11 (95% CI: 1.61–2.77) trips/hr during the night period. Night provisioners did not experience improved reproductive success, which supports our conclusion that night provisioning was compensatory rather than ‘bonus’ energy. Nocturnal provisioning was best explained by increased day length, which corresponded to increased light levels during the night period that enabled parents to navigate and locate food. Swa...