CLUTCH SIZE IN EASTERN PHOEBES (SAYORNIS-PHOEBE) .2. AGE EFFECTS AND SEASONAL-VARIATION

We examined seasonal variation in clutch size among single- and double-brooding eastern phoebes to learn how the pattern of variation among female age-classes and early and late-season breeders contributed to overall seasonal variation in clutch size. Clutch sizes of eastern phoebes did not decline predictably (linearly or quadratically) with season when we considered all clutches laid over the season, nor when the first and second clutches of double-brooding birds were each regarded separately. Total seasonal egg production by double-brooding second-year (SY) females declined linearly with season but that of after-second-year (ASY) females did not. Among single-brooding females, which were nearly all SY birds, clutch size, and hence total seasonal egg production, were unaffected by season. In general, SY birds bred later, laid fewer eggs, and were less likely to double-brood than ASY birds. The large seasonal variation in clutch size for the entire population was influenced by two different groups of SY females: (1) double-brooding SY females that nested at the same time as double-brooding ASY birds and had similar clutch sizes, but showed a linear decrease in total seasonal egg production versus initiation date, and (2) single-brooding SY females that nested after double-brooding females, had clutch sizes similar to double-brooding females, and showed no linear decrease in total seasonal egg production versus initiation date. Combined with double-brooding ASY females, these two groups of SY birds increase the variation in clutch size observed on any given day and obscure the pattern of seasonal decline commonly reported for other species.