Production of insurance eggs in Nazca boobies: costs, benefits, and variable parental quality

Abstract

Numerous studies of birds have demonstrated the value of producing insurance eggs. We have previously found that second eggs provide insurance against failure of first eggs for Nazca boobies (Sula granti), which raise only one chick to fledging, yet some females lay only one egg. We used an 8-year data set to compare 2 hypotheses for clutch size variation: one based on trade-offs, predicting declining future performance by females that lay a costly second egg and one based on parental quality, predicting that intrinsically superior females lay 2 eggs both currently and in the future. Clutch size variation did not contribute to the best multistate mark–recapture model of survival, suggesting that clutch size and survival are unrelated and that any survival cost of reproduction related to laying second eggs is small. Transition probabilities between reproductive states were generally, but not entirely, inconsistent with fecundity costs of producing a marginal egg. Parental quality effects were apparent, with females tending to remain in a given reproductive state across seasons. Parents producing a marginal egg had consistently higher breeding success than did parents of 1-egg clutches, due principally to the insurance effect and secondarily to differences in parental quality after hatching. Different model selection approaches gave differing results for logistic regression analyses of breeding success of females that hatched eggs. A conventional significance-testing, iterative-fitting approach excluded insurance state from the top model, whereas the information-theoretic (I-T) approach detected an association between current insurance state and subsequent survival and fecundity. Using the I-T approach, potentially biological significant (but not statistically significant under conventional analysis) effects were detected that may otherwise have been ignored.