I observed 12 nesting cycles of Tachycineta bicolor (Tree Swallow) with emphasis on the choice of nest materials used, timing of egg-laying, presence and behavior of conspecifics, and nesting success. All Tree Swallows built nests of dried grasses predominantly lined with large, plumed white feathers, but nest-lining behavior did not occur until after egg-laying began. When I experimentally removed feather-linings from one nest, the pair replaced them. In all nests, the birds tucked the feather quills under the eggs with the plumes arching over them. The Tree Swallows showed a strong preference both for shape of feathers and their color contrast from background. The Tree Swallows exhibited vigorous territorial behavior and nest-guarding from conspecifics before nest-building started, continued until early incubation, and then ceased when the young hatched. Birds other than the nest owners were frequent visitors in the nest area and they sometimes entered nests throughout the nesting cycle. Indirect evidence suggests that extra-pair eggs appearing in the nest and “extra” young accounted for nestling mortality . I here infer that although Tree Swallows experience strong nest parasitism, feather-lining behavior hides information on nest contents and thus reduces parasitism.