Does disturbance by nocturnal predators affect body mass of adult Common Terns?

To investigate the effects of nocturnal disturbance of predators on body co ndition of adult Common Terns (Sterna hirundo) we present data on body mass , mass change from evening to morning as well as data on flight activities and reproduction during nocturnal predation by a Long-eared Owl (Asio otus) and compare the periods before, during, and after predation. Birds were in dividually marked with transponders, allowing continuous recording of activ ity on resting sites and body mass. Predation caused prolonged incubation p eriods and lowered hatching and breeding success. Terns showed high flight activities during evenings ("panic flights") and adults left the colony one hour after sunset, performing flights through the entire night and returne d to resting sites in the morning at the time when normal flight activity s tarted in the periods without predation. In 24% of the nests, mostly outsid e of the predation center, the incubation period was not prolonged, indicat ing a return of these adults in the night to their nests. During the predat ion period, body masses were not reduced and mass loss during night (differ ence in mass between evening and morning) did not differ compared to non-pr edation periods. The only difference between periods was the lack of correl ation between evening mass and mass change during the night and a positive correlation between morning mass and mass change during night in the period when owl predation occurred. Terns appeared to forage very early in the mo rning before returning to the colony site, compensating for increased energ y expenditure after extensive flight activity. Adults chose to leave the co lony site to escape predation and to forage before returning, a strategy th at appeared to favor self-survival over survival of offspring.