Age‐Specific Patterns of Reproduction in the Glaucous‐Winged Gull: Increased Effort with Age

Abstract

I examined age—specific breeding patterns in the Glaucous—winged Gull (Larus glaucescens) in Washington and evaluated the importance of age—specific changes in reproductive effort as an explanation for improved breeding performance with age of the adult. Age was related to nest location, laying date, egg size, clutch size, hatching success, and territory size. Most variables influenced by age improved for at least the first two breeding seasons. Improved breeding performance is best explained by increased experience and, for two breeding variables, by colony growth. A decline in performance in several parameters (clutch size, egg size, and hatching success) among older birds may be a result of senescence. No evidence existed for an increase in reproductive effort with age in the Glaucous—winged Gull. During incubation, adult mass, relative to body size, was negatively correlated with age. However, the reduced mass of older birds is not evidence of increased reproductive effort for three reasons: (1) birds provided with supplemental food did not gain mass during incubation; (2) older birds were as likely as young birds to lay a fourth egg if the first was removed; and (3) birds were not limited in foraging time during incubation. During the nestling period, body mass, relative to size, was not correlated with adult age; however, older birds gained more mass than young birds between incubation and the end of the nestling period. The amount of time that adults spent foraging was unrelated to age. However, older birds spent more time resting and older females were significantly less aggressive than young females. Older birds also left the territory vacant for less time, indicative of better coordination of nesting activities. The absence of the predicted increase in reproductive effort may be due to direct effects of senescence combined with weak selection for an increase in effort.