We report a natural cross-fostering experiment of two House Wren (Troglodytes aedon) eggs and nestlings by Tree Swallows (Tachycineta bicolor). We document an increase in duration of the House Wren incubation period by 1–2 d, which closely corresponds to the mean incubation period of their hosts. We also documented nestling growth rates of both the host Tree Swallows and the alloparented House Wrens. The host parents apparently fed the House Wren nestlings; however, the wrens exhibited slow growth compared with conspecific nestlings in neighboring nests. The wren nestlings eventually died at 6 and 13 d of age. We hypothesize that the lower incubation temperatures, and later, interspecific sibling competition that the House Wren nestlings were exposed to in the Tree Swallow nest may have prolonged the incubation period and slowed the growth rates of the House Wren nestlings.