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 ch ick recruitment, leading to a strategic decrease in clutch size with l aying date. In an effort to shed light on this area of disagreement, w e analyzed the physiological condition (as measured by body mass, brea st muscle thickness, and fat stores) and body size (as measured by win g and head lengths) of 184 female Tree Swallows (Tachycineta bicolor) nesting near Ithaca, New York in 1993, 1994, and 1995, and related the ir condition to their laying date and clutch size. Through multiple re gressions, we found that female age and wing length were the only sign ificant predictors of laying date and that female age and laying date were the only significant predictors of clutch size. Thus, when the ef fects of laying date were held constant, there was no detectable effec t of condition on clutch size in this species. This result is inconsis tent with the constraining-condition hypothesis, and it suggests that the seasonal decline of clutch size in Tree Swallows is most appropria tely seen as a strategic adjustment by the female to varying prospects for her offspring. The lack of evolution in laying date in at least t he Tree Swallow remains a paradox.