Breeding success of western gulls (Larus occidentalis) on Alcatraz Island: The effects of experience, sex and condition

Margaret Elizabeth Brown, Purdue University

Abstract

Understanding patterns of phenotypic variation and underlying processes that cause them is important to the study of adaptations. Phenotypic variation can be studied by looking at both the proximate mechanisms that produce variation and their fitness consequences. In this study, I examined phenotypic variation in Western Gulls (Larus occidentalis) by studying variation as the product of interactions among several hierarchical effects. Gulls are considered ecological generalists; however, within species they exhibit considerable variation in behavior. My field work consisted of three years of monitoring diet, parental care behavior, nest microhabitat, and reproductive success of Western Gulls on Alcatraz Island, California. Western Gulls are monogamous seabirds with biparental care, site and mate fidelity, and maximum lifespan of 25 years. I examined life history effects, such as those due to age, by determining how parental breeding experience influences nest site selection, diet choice and reproductive success. The complex interactions among experience, sex and offspring behavior were studied by determining how each factor influenced parental care behavior and reproductive success. I predicted that pairs with greater experience would have higher quality diets and nesting habitat, which should result in higher fledging success. I also predicted few sex differences in behavior due to the pattern of similarity in investment between the male and female parent for monogamous species. I found that previous breeding experience affected behavior and reproductive success less than predicted, as new and established pairs showed similarity in nest site choice, feeding behavior and reproductive success. Nesting habitat was highly variable, but pairs showed similar nest site selection between years. The sexes differed in their feeding behavior, perhaps due to differential nutritional requirements or mating strategies. Although my study revealed a high degree of variation in diet, nest microhabitat and behavior among Western Gulls on Alcatraz, such variation had little effect on reproductive success. The lack of fitness effects may be the result of favorable environmental factors on Alcatraz, such as proximity to alternative food sources and low nesting densities, which may allow behavioral and ecological variation without strong reproductive success consequences.

Degree

Ph.D.

Advisors

Howard, Purdue University.

Subject Area

Ecology|Zoology

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