Southern, William E.
M.S. (Master of Science)
Department of Biological Sciences
Ring-billed gull; Birds--Behavior
Food stealing among members of the family Laridae has received considerable study in recent years (e.g. Hatch 1970; Hays 1970; Hopkins and Wiley 1972; and Dunn 1973). Most of these studies have dealt with interspecific robbing behavior where one or more gulls search for food among terns. Piracy may also occur intraspecifically as in the case of breeding Ring-billed Gulls stealing from conspecifics. In a preliminary study conducted in 1977, C. D. Rymal, W. E. Southern, and myself (Elston et al. 1977) found piracy to adversely affect the efficiency with which adult Ring-billed Gulls supplied food to their young. During the 1978 and 1979 breeding seasons I continued to study the effects of piracy on breeding Ring-billed Gulls. I monitored feeding attempts by direct observation to: (1) determine how feeding behavior changes as a consequence of piracy; (2) ascertain which factors affect the incidence of piracy; and (3) find out if reduced feeding efficiency resulting from frequent piracy places a stress on parent-chick bonds. Incidence of piracy was found to be affected by two factors: nest density, and the type of food being fed to the chicks (fish, insects, or worms). Piracy was found to alter adult behavior prior to regurgitation of food to chicks. The behavioral changes resulted in feedings taking longer and becoming less frequent. Piracy was also found to indirectly affect feeding success, causing it to drop as the breeding season progressed. I have proposed that this reduction in feeding success leads to a conflict between the chicks that are demanding more food and the adults that are becoming increasingly hesitant to feed. I believe this conflict results in the breakdown of the parent-chick bond, ultimately causing the chicks to depart from the colony. The effects of piracy on juvenile survival, after leaving the colony are also discussed.
Elston, Sue F., "Intraspecific piracy in breeding ring-billed gulls" (1980). Graduate Research Theses & Dissertations. 3630.
vi, 54 pages
Northern Illinois University
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