EFFECTS OF HANDICAPPING ON FEMALE CONDITION AND REPRODUCTION IN TREE SWALLOWS (TACHYCINETA BICOLOR)

We reduced the foraging efficiency of female Tree Swallows (Tachycinet a bicolor) during the prelaying period by removing one-third of their night feathers by clipping them at their bases. Clipped females laid l ater and smaller clutches, and their clutch sizes were reduced more th an 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. Femal es that were clipped fed their young at lower frequencies and were les s likely to return the following breeding season. There were no effect s 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 Sw allows base their early-season reproductive decisions largely on their income from foraging rather than the size of somatic stores of resour ces.