Chick growth in the California Gull: Relationships with hatching asynchrony and parental age

Chick mass and growth during a six-day period were investigated among California Gulls (Larus californicus) in 1979 and 1980. In broods of two, levels of hatching asynchrony between siblings ranging from 0 - 4 days were observed. Hatching asynchrony resulted in initial mass differences between siblings that persisted and accelerated with time. Fledging success was unrelated to hatching asynchrony but was related to large offspring mass and increased with parental age. Parental age was unrelated to hatching asynchrony but heavily influenced the brood's gain in mass. Six days after the second egg hatched, chick mass was highest at those levels of hatching asynchrony where mean age of parent was highest. Mass differences between siblings at day six were related to fledging success only to the extent that rapid growth among siblings of older parents usually resulted in large differences in mass. Implications of results to investigations of Lack's brood reduction hypothesis are discussed.