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 Tre e Swallows (Tachycineta bicolor) in a nest-box population in eastern O ntario, Canada. Females and nests were then monitored to determine whe ther a replacement clutch was laid. Forty-one percent of females laid replacement clutches. First clutches of those females that laid replac ement clutches were significantly larger than those elf females that d id not relay. This suggests that female quality, indicated by having h igher levels of expendable energy or greater foraging skill or efficie ncy, is the determining factor as to whether or not a replacement clut ch will be laid. For birds that re-laid, the replacement clutch was si gnificantly smaller than the first clutch. This suggests either a depl etion of energy reserves with each subsequent nesting attempt, or an i ndividual female's energetic tradeoffs to ensure her own survival to t he next breeding season.