Recovery of breeding success in a population of brown pelicans

Breeding populations of the brown pelican (Pelecanus occidentalis) declined during the 1960's on both east and west coasts of the United States. In 1969, colonies in South Carolina fledged an average of 0.78 young per nest (Blus et al. 1974b), and those in California only 0.004 (Anderson et al. 1975). The minimum production for population stability has been estimated to be 1.0 to 1.2 fledglings per nest (Henny 1972, Anderson et al. 1977). The South Carolina population may have decreased by as much as 80% during the previous decade (Blus 1970).

Organochlorine pesticides were implicated as a cause of population decline. Eggs from South Carolina contained an average of 5.4 parts per million (ppm) of DDE (wet weight basis) in 1969, and those from California about 70 ppm wet weight (Blus et al. 1974a, Anderson et al. 1975). Shells of South Carolina brown pelican eggs were,17% thinner than normal, and those from California were 30% thinner, compared with shells of eggs laid before DDE was introduced in 1947 (Blus et al. 1974a, Anderson et al. 1975); crushed eggs were common in the colonies. DDE was the primary cause of shell thinning (Blus et al. 1971), but dieldrin was also associated with breeding failure, and both may be embryotoxic (Porter and Wiemeyer 1969, Longcore et al. 1971, Blus et al. 1974b).

Brown pelicans also declined in Louisiana and Texas during this period, in association with shell thinning and relatively high organochlorine levels (Anderson and Hickey 1970, Blus et al. 1975, King et al. 1977). Eggs of pelicans in Florida, however, contained lower residues, and numbers have remained stable (Williams and Martin 1970). In 1969, the Patuxent Wildlife Research Center and Cape Romain National Wildlife Refuge initiated an intensive study of the South Carolina population. Size of the breeding colonies, production, eggshell thickness, and organochlorine residues were monitored each year. Between 1969 and 1976 organochlorines in the eggs declined gradually, reproductive success improved, and the breeding population doubled (Blus et al. 1979).

We have continued to monitor the South Carolina brown pelican population. Results for 1977 and 1978 are presented here, with an evaluation of the significance and possible causes of current breeding success.