Seasonal Tradeoffs in Reproductive Investment in a Multi-Brooded Passerine

Abstract

Because investment in eggs is costly for birds, a female's body condition is expected to influence that investment. Moreover, resource availability can fluctuate by year, and older and younger females may acquire or allocate resources differently. Additionally, in multi-brooded species, females may make tradeoffs in investment in first and second clutches. Because environmental conditions often change through the breeding season and early maturity of second broods can increase females' fitness, patterns of females' allocation to each clutch may change with the season. Here, we investigate associations between females' quality (body condition and age) and pre-hatching investment (clutch size, average egg mass per clutch, and total clutch mass) in adults of the Eastern Bluebird (Sialia sialis), a multi-brooded cavity-nesting passerine. In the two years of the study, relationships differed; in 2003, females that were heavier relative to their body size produced larger eggs and invested in greater overall clutch mass, and younger females produced larger clutches. In 2004, females whose first clutches were greater in overall mass laid second clutches proportionately smaller in overall clutch mass. Together, these data suggest that overall clutch mass may represent a significant cost to female bluebirds. We also found variation in investment in eggs within the season; second clutches tended to be smaller than first clutches but tended to contain larger eggs. By increasing resources per offspring late in the breeding season, females may increase the survival or condition of late-season offspring.