Status of a Northern Population of California Brown Pelicans

In the Southern California Bight (SCB), Brown Pelican (Pelecanus occidentalis californicus) breeding populations were apparently historically stable. They are again increasing after pollution-related declines in the late 1960's and early 1970's. This increase has been bolstered by recruitment into the breeding population of pelicans fledged outside the SCB area, but populations did not increase significantly until after improvements in reproduction of the SCB population itself. Nesting chronology is apparently related to food availability. Earlynesting, whether at Anacapa Island or Islas Los Coronados, usually confers a reproductive advantage. This advantage varies at each location; breeding populations reassort each year, apparently in response to variable food supplies. Shifting between two major breeding colonies in the SCB is apparently a response to a changing food supply distribution. Annual variations in normal (as opposed to pollution-affected) breeding population size result partly from changing proportions of the available adult population that actually breeds. Ornithologists continue to study the factors associated with reproduction and survival in different bird species, in different environments and at different times (see Ricklefs 1977). When populations are declining, improvements in reproduction, survival, or both, can be adequate for population recovery (see Goodman 1980). Although variations in mortality need to be understood, maximum production of young is still critical for population maintenance or increase, especially in environments where the food supply is highly mobile and varies somewhat unpredictably in availability (Cody 1971). Pollutant-related reproductive problems for California Brown Pelicans (Pelecanus occidentalis californicus) in the Southern California Bight (SCB) (reviewed below) and elsewhere in the Brown Pelican range (see Schreiber and Risebrough 1972, Schreiber 1980a), in the decade of the early-1960's to early-1970's, have clouded the understanding of changes of status in SCB pelican populations. However, unnatural catastrophies can provide a setting whereby the processes of population change and maintenance might be examined in different parts of a species' range. Here, our objectives are threefold: (1) to briefly review recent status changes of SCB pelican populations, (2) to present more detailed information on Brown Pelican demography (and as a comparison, similar data on Double-crested Cormorants, Phalacrocorax auritus) over the most recent decade or so (1969-1980), and (3) to evaluate changes in numbers and distribution and their dynamics in the SCB. STUDY SITES AND METHODS Data we discuss here were accumulated over 12 years of field work on breeding populations in the northern part of the breeding range of the California Brown Pelican. The major breeding colonies (after the definition of "colony" by Gochfeld 1980) of this population occurred on West Anacapa Island and Isla Coronado Norte located in the Southern California Bight (see Southern California Coastal Water Research Project 1973 for a definition of the SCB) (Fig. 1). Other islands in the SCB area were surveyed each year, or their status reported to us by cooperators, so that we are certain no colonies were omitted from our analysis. Breeding pelicans were observed by D.W.A. or F.G. on four to six visits per breeding season, each visit lasting from one to seven days. At the conclusion of each breeding effort for synchronous units within each colony, or at the termination of breeding for the entire colony, samples of young were banded and colormarked, addled eggs and broken eggshells collected for pollutant analyses, conditions of young determined, carcasses counted and sometimes collected, and regurgitations examined and sometimes collected. Aerial population surveys were conducted in 1972 by D.W.A. and 1979-1980 by F.G. Statistical tests used here are described by Sokal and Rohlf (1969).