The potential to lay replacement clutches by tree swallows

To determine the factors that influence a female's decision of whether or not to re-lay, we removed first clutches from 17 marked female Tree Swallows (Tachycineta bicolor) in a nest-box population in eastern Ontario, Canada. Females and nests were then monitored to determine whether a replacement clutch was laid. Forty-one percent of females laid replacement clutches. First clutches of those females that laid replacement clutches were significantly larger than those of females that did not relay. This suggests that female quality, indicated by having higher levels of expendable energy or greater foraging skill or efficiency, is the determining factor as to whether or not a replacement clutch will be laid. For birds that re-laid, the replacement clutch was significantly smaller than the first clutch. This suggests either a depletion of energy reserves with each subsequent nesting attempt, or an individual female's energetic tradeoffs to ensure her own survival to the next breeding season.