The seasonal decline in tree swallow clutch size: Physiological constraint or strategic adjustment?

The seasonal decline in clutch size has been explained as being due to either: (1) a constraining effect of female condition on both laying date and clutch size; or (2) a seasonal decline in the prospects of chick recruitment, leading to a strategic decrease in clutch size with laying date. In an effort to shed light on this area of disagreement, we analyzed the physiological condition (as measured by body mass, breast muscle thickness, and fat stores) and body size (as measured by wing and head lengths) of 184 female Tree Swallows (Tachycineta bicolor) nesting near Ithaca, New York in 1993, 1994, and 1995, and related their condition to their laying date and clutch size. Through multiple regressions, we found that female age and wing length were the only significant predictors of laying date and that female age and laying date were the only significant predictors of clutch size. Thus, when the effects of laying date were held constant, there was no detectable effect of condition on clutch size in this species. This result is inconsistent with the constraining-condition hypothesis, and it suggests that the seasonal decline of clutch size in Tree Swallows is most appropriately seen as a strategic adjustment by the female to varying prospects for her offspring. The lack of evolution in laying date in at least the Tree Swallow remains a paradox.