Prefledging survival and reproductive strategies in black brant

Abstract

We develop a general model useful for estimating survival of young waterfowl between hatching and fledging. Our model allows for interchange of individuals among broods and relaxes the assumption that individuals within broods have independent survival probabilities. We consider point estimation of survival rates and corresponding variances of the point estimators. We estimated gosling survival of black brant (Branta bernicla nigricans) during summers of 1987-89 on the Yukon-Kuskokwim Delta, Alaska. Eight-two percent of females radio-marked at hatch fledged at least 1 gosling (brood success). Survival of goslings within broods was estimated by 3 methods: (1) changes in mean brood size through time, (2) observation of goslings associated with marked adults, and (3) age ratios of brant captured in banding drives. Estimates of survival within successful broods averaged 77% and ranged from 57 to 90%. Combining brood success and survival of young within broods yields estimates of overall gosling survival which averaged 64% and ranged from 77% in 1987 to 52% in 1989. We analyzed variation in egg size of black brant in relation to clutch size, laying date, female age, year, and position in the laying sequence. Egg size increased with clutch size and female age, and decreased with laying date, year, and position in the laying sequence. We did not detect a negative phenotypic correlation between clutch size and egg size. However, overlap in total clutch volumes for clutches of different sizes indicated trade offs occurred among individuals with comparable investments in their clutches. We web-tagged black brant goslings at hatch, recorded their egg size, position in the egg-laying sequence, initial brood size, hatch date, and nesting density and examined the effect of these characteristics on their probability of recapture. Larger broods from larger eggs, and with earlier hatch dates were more likely to be recaptured. There was a tendency for young females to be less successful in rearing their broods; however, this may be related to their egg size, initial brood size, and hatch date, rather than age per se.