Effects of ENSO and PDO Events on Seabird Populations as Revealed by Christmas Bird Count Data

Abstract

--Analysis of Christmas Bird Count (CBC) data for the Honolulu, HI area showed a population which dropped following the strong 1982-1983 ENSO (El Nifio Southern Oscillation) to 4% of its pre-ENSO value for Great Frigatebirds (Fregata minor) and to 35% for Red-footed Boobies (Sula sula). The time for the populations to regain their pre-ENSO values was over a decade. CBC data for the Farallon Islands, CA shows a less striking but still significant decrease in populations of Brandt's Cormorants (Phalacrocorax penicillatus), Pelagic Cormorants (P pelagicus), and Common Murres (Uria aalge) following the 1982-1983 ENSO. Both the Hawaii and Farrallon Islands data exhibit longer term correlations with the Pacific Decadal Oscillation (PDO). Populations are suppressed during the warm phase of the PDO. CBC data for some California and Oregon coastal areas showed some weaker indications of an ENSO effect for the latter three species but little indication of a PDO effect. Received 21January 2000, accepted 20 April 2000.