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

Numerous studies of birds have demonstrated the value of producinginsurance eggs. We have previously found that second eggs provideinsurance against failure of first eggs for Nazca boobies (Sulagranti), which raise only one chick to fledging, yet some femaleslay only one egg. We used an 8-year data set to compare 2 hypothesesfor clutch size variation: one based on trade-offs, predictingdeclining future performance by females that lay a costly secondegg and one based on parental quality, predicting that intrinsicallysuperior females lay 2 eggs both currently and in the future.Clutch size variation did not contribute to the best multistatemark–recapture model of survival, suggesting that clutchsize and survival are unrelated and that any survival cost ofreproduction related to laying second eggs is small. Transitionprobabilities between reproductive states were generally, butnot entirely, inconsistent with fecundity costs of producinga marginal egg. Parental quality effects were apparent, withfemales tending to remain in a given reproductive state acrossseasons. Parents producing a marginal egg had consistently higherbreeding success than did parents of 1-egg clutches, due principallyto the insurance effect and secondarily to differences in parentalquality after hatching. Different model selection approachesgave differing results for logistic regression analyses of breedingsuccess of females that hatched eggs. A conventional significance-testing,iterative-fitting approach excluded insurance state from thetop model, whereas the information-theoretic (I-T) approachdetected an association between current insurance state andsubsequent survival and fecundity. Using the I-T approach, potentiallybiological significant (but not statistically significant underconventional analysis) effects were detected that may otherwisehave been ignored.