The ecology of Western Gulls in habitats varying in degree of urban influence

Abstract

We studied reproductive performance, diet choice, and habitat selection in Western Gulls, Larus occidentalis, over a 22-year period in five colonies along the coast of California. These included colonies with heavy (Alcatraz, Anacapa), moderate (Moss Landing), and relatively little (Southeast Farallon, Santa Barbara Island) urban influence. General aspects of habitat selection were similar on all colonies, with birds seeking nest sites sheltered from the prevailing wind, and generally avoiding grassy areas. Important differences in diet choice and habitat selection were found between urban and non-urban sites, with urban gulls relying more heavily on human refuse as food, and often locating nests adjacent to human debris and artifacts. In all colonies, Western Gulls relied heavily on fish as a primary diet item, especially when rearing offspring, and birds that took predominantly fish and other pelagic prey showed superior breeding performance. Diets of less successful birds varied among sites, with urban birds dependent on human refuse, and non-urban birds being more dependent on other seabirds and scavenging. During major El Nino/Southern Oscillation events, less successful birds on non-urban sites (i.e., Southeast Farallon) flew long distances to obtain human refuse. A captive rearing experiment in which chicks were fed diets comprised of either fish or refuse, revealed that refuse was nutritionally inadequate for normal chick development. In all colonies, only a small percentage of pairs were successful as first-time breeders, and these pairs continued to be successful over many years. In contrast, most pairs were unsuccessful in their first year, only breed for 1-2 years, and showed poor performance, on both an annual and long-term basis.