Use of Age-Classes in Monitoring Population Stability of Brown Pelicans

Abstract

The brown pelican (Pelecanus occidentalis) population on the west coast of Florida was censused for 8 years. Annual and seasonal fluctuations in total numbers of birds and age-class composition occurred. The population was highest in summer-fall and lowest in fall-winter. Adults were most common during the breeding season. The ratio of immatures to adults reflected productivity in nesting colonies and techniques for gathering accurate data are outlined. Fluctuations in the population were related to temperature, but additional data on food availability are needed to understand the population stability of this species. Total counts of individuals and age-class ratios are useful in determining population stability, but surveys of large geographic areas with sizable numbers of birds are required for accuracy. J. WILDL. MANAGE. 47(1):105-111 The brown pelican is a conspicuous and readily observed species in coastal regions of the southern United States, Central America, and north and western South America. The United States population was classified as an endangered species in the early 1970's (Schreiber 1980a) and intensive study was begun into all aspects of its reproductive biology because of concern for the continued existence of the species. As 1 aspect of an 8-year study on the biology of the brown pelican in Florida (Schreiber 1976a,b, 1977, 1979, 1980b), we gathered data on the habitat use (Schreiber and Schreiber 1982) and the size and age structure of the population. These data are vital for determining population stability, particularly in long-lived species. We also examined whether a population can be monitored over a long period of time in an inexpensive manner and without decreasing reproductive output in nesting colonies because of investigator intrusion. This work was financially and logistically supported by the Frank M. Chapman Memorial Fund, International Council for Bird Preservation, Boston Whaler Company, Outboard Marine Corporation, National Geographic Society, G. Watson, A. Connelly, S. Sprunt IV, National Audubon Society, and contributors to Seabird Research, Inc. J. Lincer and especially T. Below contributed data. Critical editorial comments by C. Braun, J. Ogden, and J. Jehl, Jr. clarified our presentation. METHODS AND STUDY AREA During 1969 and 1970 we made preliminary observations of the pelicans in Boca Ciega Bay, Tampa Bay, Florida (Fig. 1; Schreiber and Schreiber 1982: Fig. 1). We made weekly surveys of that region by small boat between January 1971 and July 1976 on a circuit that visited all mangrove (Avicennia germinans and Rhizophora mangle) keys, sand bars, and fishing piers in 2-4 hours to avoid duplicating counts of birds. At all locations we counted and classified all pelicans visible as adults, subadults, or immatures. Adults have black bellies, white or yellow heads, dark brown or white necks, and primarily gray backs and wings. Immatures, fledged birds less than 12 months old, have white bellies and brown heads, necks, and backs. Subadults, approximately 1-4 years old, are intermediate in plumage between adults and immatures, having speckled bellies and J. Wildl. Manage. 47(1):1983 105 This content downloaded from 207.46.13.184 on Fri, 29 Jul 2016 04:30:47 UTC All use subject to http://about.jstor.org/terms 106 MONITORING BROWN PELICAN POPULATION STABILITY * Schreiber and Schreiber