Nocturnal Sleep Behavior and Vigilance of Incubating Common Terns (Sterna hirundo) at Two Inland Breeding Colonies

Although sleep is necessary for maintaining physiological and cognitive function in birds, nocturnal sleep behavior has yet to be documented for terns. Nocturnal sleep behavior and vigilance of incubating Common Terns (Sterna hirundo) were explored at two colonies, Gull Island (Ontario, Canada) for six years, and Little Island (New York, USA) throughout one breeding season, using 1-min interval, time-lapse images from infrared trail cameras. Behavioral posture and vigilance (eye[s] open) of visible study birds were recorded from the images to determine if these differed between the two colonies. Terns utilized two sleeping postures, Back Sleep and Front Sleep, nearly identical to those used by gulls. Differences in the proportion of time spent sleeping between the two colonies were surprisingly large. Terns at Gull Island spent 75% less time in Back Sleep (deep-sleep posture, 7% of the night) than those at Little Island, and 64% of night with their eyes open (vs. <20% at Little Island). Differences between the study sites that may have caused this disparity include predation risk, colony size, vegetation cover and the presence of other nesting waterbirds. Apparent, long-term sleep deprivation at Gull Island may have physiological impacts. Further research is needed to establish causes and effects of differences in nocturnal sleep behavior in Common Terns.