In many species of birds, clutch size decreases as the season progresses, but this is confounded by the fact that young birds not only often have smaller clutches but also breed later than older females. We examined the effect of time of season on clutch size, hatching success, and fledging success of female tree swallows (Tachycineta bicolor) in their second year (SY) and after their second year (ASY), and compared their reproductive performance while controlling for time of season. For both SY and ASY females, clutch size decreased significantly with later first egg dates, which caused number hatched and number fledged per brood to also decline as the season progressed. However, neither the proportion of eggs hatched nor the proportion of young fledged was correlated with first egg date. The within-season decline in clutch size was not due simply to replacement clutches or female age, and we argue that it is also not due to a decrease in food abundance over the breeding season. Age-related differences in reproductive success depended on the time of season. Early in the breeding season, ASY females were superior to SY females in every measure of reproductive performance; however, there were no significant differences between age-classes late in the season. ASY females were more successful than SY females in raising clutches of five or six eggs, which were most common early in the season, but there was no difference between age-classes in raising clutches of four eggs. We suggest that an age-related difference in foraging efficiency and within-season changes in the cost of competition could explain this pattern.
From 1983 to 1986 we studied two populations of tree swallows breeding in nest boxes near the Queen's University Biological Station, Chaffey's Lock, 50 km north of Kingston, Ontario. The New Land (NL) population has grids of nest boxes in several hayfields, and the Northeast Sanctuary (NES) population has both nest boxes and natural nest sites over open water. Boxes were mounted 1.5 m high on aluminum posts, so predation rates were artificially low. Both populations have been established for at least 8 years, and the number of breeding pairs has remained relatively constant since 1982 (NL, 55--65; NES, 30--40). Data from the two populations were pooled because there were no significant differences in clutch size between the populations within age-classes and time of season classes (Kruskal--Wallis one-way ANOVA, p > 0.05).