Reproductive Commitment and Success of Cassin's Auklet

Abstract

Cassin''s Auklets at the Farallon Islands [California, USA] demonstrate plasticity in their breeding activities by responding to change in the marine environment. Major features of the reproductive cycle, including a relatively slow growth rate, long nestling period, prolonged egg-laying, wide-spread replacement clutches and presence of extensive double brooding indicate responses to a marine environment where food may be available for a longer time. The extreme variation in the dates of clutch initiation and extent of double brooding shows that this species is capable of responding rapidly to change in some proximate environmental factor, probably the food supply. The food supply of Cassin''s Auklet may vary from 1 yr to the next and within a single breeding season. Variation in timing and synchrony of egg-laying, number of clutches per year, growth rates of nestlings raised during various times of the reproductive cycle, and general annual weight trends of auklets captured from 1968-1971 are discussed. Reproductive success was lower among auklets that laid eggs later in the breeding season. Growth rates and fledgling weights were higher, and nestling periods shorter among young hatched early in the season. Gull predation is heavy where both auklets and gulls nest together, particularly where soil is shallow. Gulls exert a strong selective influence on the population.