Effects of Handicapping on Female Condition and Reproduction in Tree Swallows (Tachycineta bicolor)

We reduced the foraging efficiency of female Tree Swallows (Tachycineta bicolor) during the prelaying period by removing one-third of their flight feathers by clipping them at their bases. Clipped females laid later and smaller clutches, and their clutch sizes were reduced more than what would be expected from the normal seasonal clutch-size decline. Feather clipping reduced subsequent female condition as measured by mass, breast-muscle thickness, and fat deposits, but these effects did not become significant until after the females laid their eggs. Females that were clipped fed their young at lower frequencies and were less likely to return the following breeding season. There were no effects of female condition near clutch completion on laying date, and there was a strong effect of laying date on clutch size, with only the most ambiguous of four condition measures (body mass) having a significant, albeit weaker, effect. Coupling these results with those of another study on unmanipulated swallows, it appears likely that female Tree Swallows base their early-season reproductive decisions largely on their income from foraging rather than the size of somatic stores of resources.