Nocturnal behaviour of breeding Common terns

Breeding Common terns Sterna hirundo typically remain at the colony at night, yet nocturnal colony desertion may occur due to predation, nocturnal foraging, or prospecting. We used GPS telemetry data to investigate nocturnal colony attendance and movements of Common terns at multiple freshwater and marine colonies. Across most colonies, nighttime colony attendance was 98%; however, at one marine colony, extremely low attendance occurred (< 0.02%), probably due to nocturnal predation. Terns from the marine colony were also recorded flying slowly over the sea at night, presumably foraging. Nocturnal prospecting of conspecific colonies beyond typical foraging ranges occurred in both sexes. Night-time tracking may help to identify previously unknown colonies and improve monitoring, especially in populations that frequently change breeding locations. For primarily diurnal foragers, including nocturnal tracking in future studies can be used to increase monitoring efficiency, identify threats, and support planning of conservation activities.