Effect of Feathers as Nest Insulation on Incubation Behavior and Reproductive Performance of Tree Swallows (Tachycineta bicolor)

-Many species of birds line their nests with feathers, presumably because of the insulative qualities of feathers and because feathers may act as a barrier between nest parasites and nestlings. In 1993, we experimentally examined the role of feathers as nest insulation on the incubation behavior, nestling growth, and reproductive performance of Tree Swallows (Tachycineta bicolor) nesting in boxes in western Michigan. There were no significant differences between the incubation rhythms of females with experimental nests (i.e. no feathers) and females with control nests (i.e. with feathers). Nestlings that were reared in control nests had significantly longer right tarsi and right wing chords; their masses were significantly greater than nestlings reared in experimental nests. In addition, nested analyses of variance indicated that both female age class (i.e. second year, after second year, or after hatching year) and the brood within which a nestling was reared had significant effects on nestling growth until nestling day 12. Whether an individual nestling was infected with ectoparasites was independent of whether it was reared in an experimental or control nest. Nest insulation affected reproductive performance: females with experimental nests had significantly longer incubation periods and produced significantly fewer fledglings than did females with control nests. These results suggest that nest insulation may be an important factor influencing incubation behavior, nestling growth, and reproductive performance of Tree Swallows in western Michigan. Received 21 November 1994, accepted 21 June 1995. NESTS ARE an important component of parental effort in birds. Nests provide safety from predators, and a container and microenvironment for incubating parents, developing eggs, and nestlings (Collias and Collias 1984). Nest building is influenced by "instinct" and learning: the nests of experienced birds often are superior to those built by novices (Collias and Collias 1984). Determining the consequences of individual differences in nest architecture on reproductive performance are important to a complete understanding of the factors affecting the evolution of parental effort in birds. Tree Swallows (Tachycineta bicolor) build nests made of a mat of dry grasses with a nest cup lined with feathers. Females do most of the nest building (Kuerzi 1941, Sheppard 1977), but males add many of the feathers that line the cup (Sheppard 1977, Cohen 1985). The nests of second-year females are less well feathered than nests built by after-second-year females (Lombardo 1994). Differences in nest feathering may influence the age-related differences in reproductive performance that have been observed in Tree Swallows (DeSteven 1978, 1980, Stutchbury and Robertson 1988) because of the effects of feathers on the thermoenergetics of incubation, brooding, and nestling growth. Winkler (1993) experimentally examined the effect of feathers as nest insulation on reproductive performance in Tree Swallows nesting in boxes near Ithaca, New York, and found that feathers as nest lining had no effect on fledging success, but nestlings reared in control nests (i.e. with feathers) were larger on the twelfth nestling day, fledged earlier, and had fewer ectoparasites than those reared in experimental nests (i.e. without feathers). He did not examine whether there were female age-related differences in reproductive performance that were influenced by feathers as nest insulation. Lombardo (1994) did a correlational study of nest architecture, female age, and reproductive performance of Tree Swallows nesting in southeastern Michigan and found no relationship between the amount of nest feathering and the number or proportion of hatchlings and fledglings produced. However, well-insulated nests were advantageous early in the season when ambient temperatures were low and eggs and nestlings needed to be kept warm, but disadvantageous late in the season when ambient