Rearing environment and offspring traits

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Early life experience can have important and long-lasting consequences. For young animals, one potential source of stress early in life is increased family (brood) size. In biparental animals, the level of parental care provided to young is an important factor in shaping the rearing environment, particularly when parents are raising many offspring. Nearly 70 years ago Lack (1947) predicted that clutch (and thus, brood) size is limited by the ability of parents to provid care to offspring, but parents can also incur costs when they increase effort for larger broods. Using a brood size manipulation experiment, I found that adult bluebirds raising enlarged broods adjusted their nest visit rates, making more visits to the nest than adults raising reduced broods. As a result, there were no differences across manipulated broods in per capita nest visit rates. Offspring raised in enlarged broods did not incur many costs in terms of growth, corticosterone (stress hormone) levels, and telomere lengths. That adults raising enlarged broods compensated by adjusting nest visit rates suggests that the benefits of fledging more young from their current brood outweighed the potential costs to future reproduction. High food availability during the year of the study may have allowed parents to increase effort for larger broods.