ABSTMACT.-Nests are an important component of parental effort in birds. From 1988 to 1990, I compared the architecture of nests built by subadult and adult female Tree Swallows (Tachycineta bicolor) in boxes in southeastern Michigan in order to determine if there were age-related differences in nest architecture and if these differences were associated with agerelated differences in reproductive performance. Nests built by Tree Swallows were composed of a mat of dry grasses and a nest cup lined with feathers. On average, nest material filled approximately 30% of a nest box. I observed few age-related differences in nest architecture. However, nests built by subadult females were significantly less well feathered than nests built by adult females, but feather score was not correlated with the production of nestlings or fledglings. There were age-related and within-season variations in reproductive performance associated with variations in nest architecture, especially later in the season. Among subadult females that bred later in the season (i.e. after the population median date of clutch initiation), those females that built larger nests (as indicated by total nest volume, depth of material beneath the nest cup, and percentage of nest box filled with material) hatched more eggs per clutch. In 1990, subadult females that bred later in the season and built larger nests (as indicated by depth of material beneath nest cup and percentage of nest box filled with material) produced significantly more hatchlings but significantly fewer fledglings. Among adult females that bred later in the season, those females that built nests that filled a larger percentage of the nest box hatched fewer eggs. Adult females with nests having larger cups produced significantly more fledglings, but those with cups crowded with nestlings (as indicated by a cup index [total egg volume/nest cup volume]) produced significantly fewer fledglings. In 1990, adult females that bred later in the season and built larger nests (as indicated by depth of nest material, total nest volume, percentage of nest box filled with material) and had more-crowded nest cups produced fewer hatchlings and fledged a significantly smaller proportion of hatchlings. These results suggest that nestling hyperthermia in well-insulated nests may affect the reproductive performance of Tree Swallows that breed late in the season. Received 11 January 1993, accepted 1 July 1993.