Foraging Behaviour in Relation to the Lunar Cycle by Australian Owlet-nightjars Aegotheles cristatus

Abstract

Summary: The purpose of our study was to assess the influence of moonlight on the foraging behaviour of Australian Owlet-nightjars (Aegotheles cristatus), an Australasian caprimulgiform bird. Australian Owlet-nightjars should face a distinct tradeoff between increasing activity during moonlit nights to enhance foraging, while at the same time risking increased exposure to visually- oriented nocturnal predators. Our objectives were to quantify foraging activity in relation to moonlight using radio telemetry and document the principal foraging strategy of Australian Owlet-nightjars by observing light-tagged, free-ranging birds. Our results showed that like true nightjars, Australian Owlet-nightjars are almost exclusively sally-type foragers and rarely forage on the ground. They suffer high rates of predation relative to other adult caprimulgiforms and are active almost exclusively during true night. Overall, activity levels inferred from telemetry data declined over the course of the study (September–March), although this effect was overridden on full moon nights when there was no seasonal effect and activity levels remained constant. Unlike true nightjars for whom bright lunar light enhances foraging activity, our results for Australian Owlet-nightjars suggest that the risk of predation influences foraging activity, especially on full moon nights. Although Australian Owlet-nightjars apparently employ the same foraging mode as nightjars do, knowl-edge about foraging strategy alone is apparently insufficient to predict the bird’s response to lunar condition. More data on activity at different times of the year combined with measurements of prey availability and information on the timing and identity of predators are needed to allow a definitive explanation for the influence of lunar light on these birds.