| Popis: |
The feeding behaviour of wintering herring gulls, Larua argentatus, at refuse tips was investigated. Immature herring gulls were less successful than adults in independent foraging and in competitive interactions. They relied more heavily than adults on stealing and attacking to obtain food. Foraging ability Improves over at least the first four years of life. Three types of foraging at a refuse tip were distinguished: a) undisturbed primary feeding which was highly competitive, b) disturbed primary feeding in which gulls dipped for food from the air and therefore a high degree of manoeuvrability was required and c) secondary feeding which involved birds feeding at low densities on dispersed and partially covered refuse. Adult male herring gulls took part in more undisturbed feeding, where their greater size afforded them an advantage in competitive interactions, whereas females opted for more disturbed feeding, where they were advantaged by their smaller size and therefore lower wing loading. The feeding methods used by the smaller numbers of great black backed L. marinus and black headed gulls, L. ridibundus present were also related to differences in competitive ability and manoeuvrability, both associated with body size. Use of kleptoparasitism increased within creasing body size. Both herring and great black backed gulls preferentially attacked aggressive birds. Flock composition was variable such that when increased numbers of great black backed gulls were present, there was also a shift towards the larger individuals of both species This constituted an increase in the frequency of kleptoparasitic individuals and under these circumstances female herring gull feeding performance was depressed, whereas that of male herring gulls and of great black backed gulls increased. Complex inter- and intra specific interactions and the high degree of skill Involved in feeding at a refuse tip led to large variation In the ability of individuals to exploit refuse as a food resource. |
