NEST ARCHITECTURE AND REPRODUCTIVE-PERFORMANCE IN TREE SWALLOWS (TACHYCINETA BICOLOR)

Nests are an important component of parental effort in birds. From 198 8 to 1990, I compared the architecture of nests built by subadult and adult female Tree Swallows (Tachycineta bicolor) in boxes in southeast ern Michigan in order to determine if there were age-related differenc es in nest architecture and if these differences were associated with age-related differences in reproductive performance. Nests built by Tr ee 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 architectur e. However, nests built by subadult females were significantly less we ll feathered than nests built by adult females, but feather score was not correlated with the production of nestlings or fledglings. There w ere age-related and within-season variations in reproductive performan ce associated with variations in nest architecture, especially later i n the season. Among subadult females that bred later in the season (i. e. after the population median date of clutch initiation), those femal es that built larger nests (as indicated by total nest volume, depth o f material beneath the nest cup, and percentage of nest box filled wit h material) hatched more eggs per clutch. In 1990, subadult females th at bred later in the season and built larger nests (as indicated by de pth of material beneath nest cup and percentage of nest box tilled wit h material) produced significantly more hatchlings but significantly f ewer fledglings. Among adult females that bred later in the season, th ose females that built nests that filled a larger percentage of the ne st box hatched fewer eggs. Adult females with nests having larger cups produced significantly more fledglings, but those with cups crowded w ith nestlings (as indicated by a cup index [total egg volume/nest cup volume]) produced significantly fewer fledglings. In 1990, adult femal es that bred later in the season and built larger nests (as indicated by depth of nest material, total nest volume, percentage of nest box f illed with material) and had more-crowded nest cups produced fewer hat chlings and fledged a significantly smaller proportion of hatchlings. These results suggest that nestling hyperthermia in well-insulated nes ts may affect the reproductive performance of Tree Swallows that breed late in the season.