Brown Pelican Foraging Success and Kleptoparasitism by Laughing Gulls

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Abstract
Previous studies have shown age-related differences in feeding success for a number of bird species, particularly those found in marine environments. We compared the foraging success of adult and immature Brown Pelicans (Pelecanus occidentalis) feeding along the Playa Miramar northwest of Manzanillo, Colima, Mexico. On many of the feeding dives, Laughing Gulls (Larus atricilla) were attracted to pelicans and often behaved aggressively in attempts to obtain food from them. Adult Brown Pelicans were significantly more successful than immatures. Laughing Gulls were attracted to successful pelicans regardless of the age of the pelican. The proportion of unsuccessful immature pelicans not attracting gulls was higher than that of unsuccessful adult pelicans. Particularly inept immature pelicans are probably easily identified by gulls and ignored completely. Although we found no differences in the frequency of pelican-gull physical contact (which we used as a measure of gull aggressiveness) between immature and adult pelicans, such behavior was directed almost exclusively toward successful pelicans. Gull aggression was also more intense when pelicans were attacked by groups of gulls.