Effects of Parental Age on Hatching Asynchrony, Egg Size and Third-Chick Disadvantage in Western Gulls

Abstract

We examined the relationships among parental age, hatching asynchrony, egg size, and the "third-chick disadvantage" of Western Gulls (Larus occidentalis) on Southeast Farallon Island, California, during the 1984-1990 breeding seasons. Third chicks showed lower survival than first or second chicks for all parental age classes combined. Fledging success of first- and second-hatched chicks was related to parental âge in a parabolic fashion, whereas fledging success of the third chick increased sigmoidally with increasing age; the probability of fledging of first-, second-, and third-hatched chicks converged for birds greater than 15 years old, negating the third-chick disadvantage for the oldest parents. Egg volume increased with increasing age for females 4 to 13 years old, and levelled off or declined slightly thereafter. For older females, the size disadvantage of third-laid eggs was relatively less than for younger females. Egg volume did not affect fledging success when parental age and year were considered in a multiple logistic (binomial) regression analysis. Hatching asynchrony decreased slightly with increasing parental age, Fledging success of third-hatched chicks decreased significantly with increasing hatching asynchrony. The greater hatching synchrony in broods of older parents may increase the competitive ability of third chicks. The third-chick disadvantage, often viewed as an adaptive response to variable food supplies, may be a nonadaptive effect of resource availability governed, in part, by the foraging efficiency and/or reproductive effort of the parents.