Summary
We investigated the division of labor in male and female tree swallows by measuring the rate of food delivery to nestlings at 36 nests. By observing natural nests and performing brood manipulations we found that males and females divided the feeding duties about equally and responded similarly to changes in brood size and age. Feeding rate was most highly correlated with brood mass. Manipulation and removal experiments showed that increased feeding rates could be elicited, but only for limited periods of time. Male and female tree swallows could only partially compensate in feeding nestlings when mates were removed. This, along with the higher mortality in enlarged broods and in those raised by single parents, indicates that both male and female are required to raise an entire brood to fledging. We argue that this requirement contributes to the absence of mate guarding and the maintenance of monogamy in the tree swallow.
Methodology
This study was done during the summer of 1982 at the Queen's University Biological Station near Chaffey's Lock, Ontario (44°34' N, 76°20' E). The study area consisted of seven hayfields and a nearby marsh in which grids of nest boxes had been established in 1977 or earlier. All boxes were mounted on aluminum poles to prevent predation on the nest. Of 139 boxes, 96 were arranged in spiral cells of six boxes each, with boxes located 1, 2, 4, 8, and 16 m from a central box. The other 43 boxes were in a rectangular grid with 25 m spacing, or as single boxes in isolated locations as described in Leffelaar and Robertson (1984). Fifty-five clutches were initiated in the study area in 1982. Usually only one box within each spiral was occupied because tree swallows defend an area of about 15 m radius around their nest site (Robertson and Gibbs 1982; Muldal et al. 1985).