Causes and consequences of within‐population variation in inter‐brood interval of a migratory songbird

Many birds produce two broods each year and exhibit significant intra‐population variation in the length of time between the hatching of first and second broods (the inter‐brood interval). Why within‐population variation in inter‐brood interval occurs and whether longer inter‐brood intervals influence second‐brood survival remain key questions in understanding the benefits of producing a second brood. We examined the causes and consequences of variation in inter‐brood interval in a wild population of migratory Savannah Sparrows Passerculus sandwichensis breeding on Kent Island, New Brunswick, Canada. Using a 29‐year dataset, we show that females tended to have short inter‐brood intervals when their first nest was initiated later in the season, when they had a low number of first‐brood fledglings to care for, or when they had higher lifetime recruitment (suggesting that they were of higher quality). Males with the shortest and longest inter‐brood intervals had low feeding rates. Finally, structural equation modelling provided evidence that longer inter‐brood intervals led to later second‐brood hatch dates, which in turn led to smaller clutch sizes and fewer recruits. Our results suggest that the length of time between broods is a key factor predicting the first‐year survival of second‐brood offspring. These long‐term data reveal that inter‐brood interval reflects the timing of breeding and variation in female quality and that it influences offspring survival and, likely, fitness.