Brown Pelicans: Influence of Food Supply on Reproduction

From 1970 through 1979, availability of northern anchovies Engraulis mordax, the major food source for breeding brown pelicans Pelecanus occidentalis californicus, was the most important determinant of fledging success. Once the birds are constrained to a breeding area, a weak relationship exists between large scale indices of anchovy abundance in the Southern California Bight (about 105 kM2) and mean pelican fledging rates. This relationship becomes much stronger if pelican feeding behavior and more localized areas associated with their breeding colonies (about 8 x 103 kM2) are considered separately. Anchovy abundance over small areas (about 2 x 103 km2) showed no relationship to pelican reproduction. This suggests that events which influence large geographic areas (such as weather-patterns) affect reproduction through prey levels. Yet, pelican reproductive output is more precisely related to local aspects of prey availability and abundance. Fledging rates are a suitable indicator of local food conditions because pelicans are constrained to more local feeding areas during breeding. High levels of annual variation in food supply result in high annual variations in pelican reproductive performance. Anchovy densities are greater near the two island areas where pelicans most commonly breed than mean density over the entire Southern California Bight. Other regions of high density in prey also exist during the pelican breeding season but they are probably not available to breeders. The locations of traditional breeding colonies most likely represent environmental situations where the combination of nesting substrate and attainable food supplies occur together with consistency.