Gulls of a feather do not sleep whenever—circadian rhythm of activity of black-headed gulls Chroicocephalus ridibundus during the incubation period

Abstract Many vertebrates exhibit a diel activity, steered by light–dark cycle. However, some colonial waterbirds, in that several species of gulls, are active not only day hours but also at night. In this study, we aimed to investigate 24 h cycle black-headed gulls (BHG) Chroicocephalus ridibundus activity with focus on sleep behaviour. We expected patterns differ between colonies located various habitats, and within colony nests the centre vs edge. studied behaviour based 9600 30 s videos from camera-traps taken six data 10 GPS-tracked individuals one recorded during incubation. BHGs stayed average 48.1% night, mainly spent passive nest defence, maintenance. similar time these activities hours. Individuals breeding slept night longer than those its 76.5% daytime. two urban highest densities level light pollution birds less other (three rural urban) characterized lower intensity after sunset near colony. Knowledge nocturnal is crucial comprehend an organism, especially understand flexibility for restoration, like sleep.