Brown Pelicans in Southern California: Habitat Use and Environmental Fluctuations

Abstract

THE CONDOR JOURNAL OF THE COOPER ORNITHOLOGICAL SOCIETY Volume 83 Number 1 February 1981 Con<Uw 83: 1-15 ©The Cooper Ornithological Society 1981 BROWN PELICANS IN SOUTHERN CALIFORNIA: HABITAT USE AND ENVIRONMENTAL FLUCTUATIONS KENNETH T. BRIGGS DAVID B. LEWIS WILLIAM BRECK TYLER AND GEORGE L. HUNT, JR. ABSTRACT.-The population size, distribution, and habitat affinities of Brown Pelicans (Pelecanus occidentalis) were investigated during 1975-1978 using monthly aerial, ship, and ground surveys throughout the Southern Cal- ifornia Bight. Pelican numbers are lowest during spring, when most birds are found near the relatively small nesting colonies at Anacapa Island and Islas Los Coronados. The annual post-breeding migration from Mexican colonies augments the local population during summer and autumn; estimated peak populations of 65,000 to 94,000 birds occurred in September and October. Throughout autumn and early winter, pelicans regularly occurred as far off- shore as Cortes Bank, 75 km seaward from the nearest island and 175 km off the mainland. Adults predominated offshore and near the California Channel Islands all year, but immature birds outnumbered adults along the mainland shore during autumn and winter. Brown Pelicans occtirred at sea in highest densities in areas of shallow, warm water within about 30 km of shore. During August and December, however, migrating birds showed no discernible affinity for particular sets of environmental conditions. The distribution of spawning northern anchovies, the principal prey of Brown Pelicans during the nesting season, was not a good indicator of bird density distribution. The effects of a two-year warming trend on pelican numbers are discussed. The California Brown Pelican (Pelecanus occidentalis) breeds primarily on islands in the Gulf of California and along the west coast of Baja California (A.O.U. 1957). The northernmost nesting location is presently Anacapa Island (34°00'N, ll9°28'W), al- though pelicans nested at Pt. Lobos, Mon- terey Co. (36°35'N, 121°58 'W) as late as 1959 (Baldridge 1974). Much research has b een directed towards documenting pelican population status in western North America, where a decline in nesting population size in the northern colonies coupled with very low reproductive output at several colonies has been linked with pollution by chlori- nated hydrocarbons (Risebrough et al. 1971, Jehl 1973, Anderson and Anderson 1976) and human intrusion into colonies (Jehl 1973, Anderson and Keith in press). These studies provide a record of nesting behavior and phenology, reproductive success and colony status during 1968-1978, but they furnish no systematic estimates of the total numbers of birds present or of patterns of habitat use away from the colonies. Our pri- mary purpose here is to provide information on population size and distribution of the Brown Pelican in the Southern California Bight (SCB) based on data gathered during the period 1975-1978. We also discuss the occurrence of pelicans in relation to ocean- ographic events in the SCB. STUDY AREA AND METHODS ST UDY AREA The Southern California Bight and the section of the California Current immediately to the west include approximately 90,000 km• of open ocean and coastal [l)