Response of the Tree Swallow (Iridoprocne bicolor) to manipulation of clutch size was studied at the University of Michigan Biological Station located near Pellston, Michigan. Daily mean nestling weights and weights of adult females were taken on 35 nests during the 1974 breeding season in an attempt to test the hypothesis that clutch size is closely linked with the ability of the adults to feed the young. The modal natural clutch size of the study population was equally five and six. For this reason, the two control groups and the two groups of manipulated clutches were designated as follows: natural clutches of five, natural clutches of six, clutches of five raised to six, and clutches of six raised to eight. Hatching success was high (11.2% failure) insuring that incubated clutches were never completely unproductive. Incubation time tended to decrease with increasing clutch size to six eggs, then increase again with larger clutch sizes. The nestling period appeared to increase in broods larger than four birds. There were no indications of a correlation between clutch size and fledging success. No significant differences were found in nestling weight and growth rate in relation to clutch size, and conflicting results on limited data give no evidence of differential adult weight-loss related to clutch size. These results are discussed in terms of the birds’ phenotypic response to a better than average year and the significance of that response.