Feeding Strategies of Male and Female Adult Herring Gulls (Larus Argentatus)

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Abstract
This paper examines the foraging behaviour of adult male and female herring gulls at refuse tips during winter. Although the majority of herring gulls were either on the tip or nearby for much of the daylight period, an individual male or female fed at the tip for only 30 minutes per day. Three types of feeding were distinguished: (a) undisturbed primary feeding on freshly dumped refuse; (b) disturbed primary feeding where the gulls fed whilst a bulldozer was moving the refuse; and (c) secondary feeding on dispersed refuse partially covered with earth. During primary feeding the herring gulls fed at high density and the feeding flock comprised 73% adults whilst during secondary feeding they were at low density and the flock contained only 25 % adults. A comparison of disturbed and undisturbed primary feeding showed that the birds dipped for food from the air in the former but searched for food on the ground in the latter. This resulted in eight times more encounters per individual and much greater competition in undisturbed feeding. Proportionally more adult male than female herring gulls participated in undisturbed (i.e. competitive) feeding, whereas the reverse was true of disturbed feeding. These differences were greatest in the first half of the winter (i.e. October-mid-December). There were no differences in the sex-ratio of those participating in secondary feeding. During undisturbed primary feeding adult females had higher pecking and walking rates and lower encounter rates than males. Males obtained at least a fifth of their food by attacking and displacing other feeding herring gulls, three times more than females. Female herring gulls fed more often on the edge of the refuse pile where competitive interactions were less frequent but also where foraging success was lower. As the undisturbed feeding bout progressed, attack rates of males increased and their swallowing rate was sustained. In contrast, the attack rates of females decreased as did the swallowing rates. The proportion of females present also decreased, suggesting that some females were responding to the increased levels of aggression by leaving the feeding area. Refuse tips provide a variety of feeding opportunities. In a competitive feeding situation the smaller female herring gulls are disadvantaged by the dominance of males. In disturbed feeding they are able to compensate to some extent because their smaller size results in greater manoeuvrability. This paper examines the foraging behaviour of adult male and female herring gulls at refuse tips during winter. Although the majority of herring gulls were either on the tip or nearby for much of the daylight period, an individual male or female fed at the tip for only 30 minutes per day. Three types of feeding were distinguished: (a) undisturbed primary feeding on freshly dumped refuse; (b) disturbed primary feeding where the gulls fed whilst a bulldozer was moving the refuse; and (c) secondary feeding on dispersed refuse partially covered with earth. During primary feeding the herring gulls fed at high density and the feeding flock comprised 73% adults whilst during secondary feeding they were at low density and the flock contained only 25 % adults. A comparison of disturbed and undisturbed primary feeding showed that the birds dipped for food from the air in the former but searched for food on the ground in the latter. This resulted in eight times more encounters per individual and much greater competition in undisturbed feeding. Proportionally more adult male than female herring gulls participated in undisturbed (i.e. competitive) feeding, whereas the reverse was true of disturbed feeding. These differences were greatest in the first half of the winter (i.e. October-mid-December). There were no differences in the sex-ratio of those participating in secondary feeding. During undisturbed primary feeding adult females had higher pecking and walking rates and lower encounter rates than males. Males obtained at least a fifth of their food by attacking and displacing other feeding herring gulls, three times more than females. Female herring gulls fed more often on the edge of the refuse pile where competitive interactions were less frequent but also where foraging success was lower. As the undisturbed feeding bout progressed, attack rates of males increased and their swallowing rate was sustained. In contrast, the attack rates of females decreased as did the swallowing rates. The proportion of females present also decreased, suggesting that some females were responding to the increased levels of aggression by leaving the feeding area. Refuse tips provide a variety of feeding opportunities. In a competitive feeding situation the smaller female herring gulls are disadvantaged by the dominance of males. In disturbed feeding they are able to compensate to some extent because their smaller size results in greater manoeuvrability.