Corticosterone predicts double-brooding in female savannah sparrows (Passerculus sandwichensis).

Given that double-brooding (rearing two broods within a season) can increase annual fecundity, it is unclear why some females in multi-brooded populations rear only one brood per season. The Quality Hypothesis proposes that double-brooded females are high quality and, thus, have sufficient energetic resources available to bear the costs of rearing two broods per season. Glucocorticoids - endocrine hormones that have a critical role in energy regulation - could reflect female quality, and, therefore, also have the potential to indicate whether a female will rear a second brood.

Reduced fitness of secondary females in a polygynous species: a 32-yr study of Savannah sparrows.

The evolution of mating systems reflects a balance of the often-conflicting interests of males and females. Polygyny, a mating system in which males have multiple mates, presents a fitness benefit to males, but the consequences for females are less clear. Females with polygynous social mates may suffer reduced fitness, especially secondary females who typically receive less male support.