Abstract
Sexual differences in the behavior of adult Great Black-backed Gulls (Larus marinus) were examined during the pre- and post-hatch periods of the 1980 breeding season. Following clutch completion, females invested more time than males in both territorial attendance and incubation, but there were no sexual differences in egg-shifting or grass-collection. In contrast, males engaged in more agonistic behavior and long-called and yelped in all contexts more than females before chick hatching. Males also displayed more mate-oriented mews and chokes than did females at this time. During the post-hatch period, there were no sexual differences in territorial attendance, brooding, or frequency of chick feedings, but males continued to exhibit higher frequencies of agonistic acts than did females, as well as higher levels of long-calls and yelps. After chick hatching, both sexes demonstrated increased levels of agonistic acts, long-calls, yelps, mews, and yeows, while males exhibited decreased frequencies of chokes and head-tosses. The results of this study strongly suggest that parental investment by male L. marinus may equal that of females.