Is reproduction by Tree Swallows cost free?

We manipulated brood sizes of Tree Swallows (Tachycineta bicolor) in 1996 and 1997 to test for the existence of intra- and intergenerational costs of reproduction. Modal clutch size was six eggs, but experimental brood sizes ranged from two to nine young. Nestling starvation was higher in 1996 (and dependent on brood size) than in 1997, but in both years enlargement of brood size resulted in increased productivity. Nestling mass near fledging was negatively correlated with brood size, but tarsus length and wing chord were not. Food deliveries by parents increased steadily between broods of two to six young but then remained constant between broods of six to nine young. The loss of female mass between incubation and the end of the nestling period was positively related to the pair's total feeding effort, and female mass near fledging declined with increasing brood size. The latter decline disappeared, however, when broods of nine were omitted. Adult return rate (1996 to 1997) was highest among birds that raised enlarged broods. Our results, and a review of other studies of Tree Swallows, suggest that broods of seven or eight young can be raised without costs to the parents or young, and it appears that costs associated with feeding young have not influenced annual fecundity of Tree Swallows. Rather, egg production is most likely limited by energy availability to laying females. A major cost of reproduction for Tree Swallows probably arises from nest-site competition in that early arrival in spring to obtain nest sites exposes adults to high risks of death from starvation.