Multigenerational fitness outcomes of double-brooding: a 30-year study of a migratory songbird

In birds, rearing multiple broods per season can substantially increase the annual number of fledglings produced. However, the contribution of double-brooding to lifetime fitness is unclear because the number of recruits arising from single- and double-brooded females is rarely measured. Poor estimates of fitness also make it challenging to document potential trade-offs between double-brooding and survival or future reproductive output. To understand the contribution of double-brooding to lifetime fitness and whether double-brooding was associated with life-history trade-offs, we used 30 years of reproductive data on female Savannah sparrows (Passerculus sandwichensis) breeding on Kent Island, New Brunswick. Estimates of fitness included an analysis of recruitment of both F1 (first generation) and F2 (second generation) offspring from females that did and did not raise a second brood. We detected no net costs of double-brooding. Double-brooded females had higher annual apparent survival rates than single-brooded females and F1 offspring from first broods of double-brooded females were more likely to recruit into the population than F1 offspring from single-brooded females. Double-brooding also improved lifetime fitness. Recruitment of F1 offspring was positively related to the number of seasons that a female double-brooded and, as a result, there was a higher number of F2 recruits from F1 offspring arising from double-brooded females than from F1 offspring arising from single-brooded females. Our results provide strong evidence that double-brooding is a beneficial reproductive strategy for Savannah sparrows and suggests that double-brooding females are likely high-quality individuals capable of rearing two broods a season with no net fitness costs.