Sexual Differences in the Behavior of Adult Great Black-Backed Gulls (Larus marinus) during the Pre-and Post-Hatch Periods

--Sexual differences in the behavior of adult Great Black-backed Gulls (Larus marinus) were examined during the preand 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 grasscollection. 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 mateoriented 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 hat parental investment by male L. marinus may equal that of females. Received 10 August 1981, accepted 28 May 1982. GVLL species (Laridae) are generally monogamous, and members of both sexes exhibit the same behavior patterns related to parental care during the breeding season (e.g. incubation, brooding, chick feeding, territorial attendance and defense). Male and female gulls are often difficult to distinguish visually, and many descriptions of their behavioral ecology have simply "lumped" sexes in data analysis. Recent studies, however, have shown that definite differences are evident in the behavior of male and female larids. For example, Tinbergen (1956, 1960) reported that male Herring Gulls (Larus argentatus) were more active in collecting nest material and were involved in territorial defense more often than females. Similarly, male and female Laughing Gulls (L. atricilla; Burger and Beer 1975) and Ring-billed Gulls (L. delawarensis; Southern 1981) have been observed to exhibit differences in agonistic behavior toward territorial intruders. Male Western Gulls (L. occidentalis) are not only responsible for the bulk of territorial defense, but also feed chicks significantly more often than do females (Pierotti 1981). Burger (1981a) reported that, although adult male and female Herring Gulls contribute equal parental investment in the post-hatch period, sexual differences in investment patterns developed as chicks grew older. Even sexual differences in spiteful and altruistic behavior have been observed in both Herring and Western gulls (Pierotti 1980). Clearly, the investment patterns of adult larids vary qualitatively, if not quantitatively, and additional data are needed to evaluate the relative significance of male and female contributions in terms of parental effort. We examined the behavior of adult Great Black-backed Gulls (L. marinus) during the incubation and post-hatch phases of the reproductive cycle. The specific objectives of this study were to: (1) investigate possible sexual differences in a number of agonistic and nonagonistic behaviors exhibited by breeding adults and (2) determine how the transition from the preto post-hatch periods influenced the behavior of males and females.