Offspring mass variation in tree swallows: A case of bet-hedging?

The evolution of reproductive strategies is affected by the ability of organisms to deal with future environmental conditions. When environments are temporally unpredictable, however, it is difficult to anticipate optimal offspring phenotype. Diversification of offspring phenotypes, a strategy called diversified bet-hedging, may allow parents to maximize their fitness by reducing between-year variation in reproductive success. The link between diversification of offspring phenotypes and individual reproductive success, however, has rarely been documented empirically. We used an eight-year dataset (1215 broods, 870 females) on individually marked tree swallows (Tachycineta bicolor) to assess whether intra-brood mass variation was compatible with a diversified bet-hedging strategy. Intra-brood mass variation was weakly, but significantly repeatable within females, suggesting consistent individual differences. Greater intra-brood mass variation, however, was not associated with reduced between-year variation in reproductive success or increased female reproductive success. Moreover, contrary to diversified bet-hedging expectations, fledging success of large broods was greater when hatchlings had similar rather than variable masses. Our results suggest that intra-brood mass variation may not result from diversified bet-hedging, but rather from complex interactions between environmental, brood, and maternal characteristics.