The Role of Cavity Size in the Evolution of Clutch Size in Tree Swallows

Abstract

The individual-optimization hypothesis states that parents lay a clutch size that maximizes their own individual recruitment (Pettifor et al. 1988). Pettifor et al. (1988) found that when Great Tit (Parus major) nests were manipulated to an equal brood size, tits that began with a larger clutch size had higher recruitment than those that began with smaller clutches. They concluded that females lay the size of clutch that is optimal for their ability or quality as parents and that this clutch size varies as a result of intrinsic and extrinsic differences among individuals. In holenesting species, optimal clutch size may be selected to correspond to the maximum number of nestlings that can fit in a cavity at the least cost to reproduction. In this case, the intrinsic difference is parental quality, and the extrinsic difference is cavity size. This hypothesis has been supported by studies showing that clutch size is positively correlated with cavity size in hole-nesting passerines (Karlsson and Nilsson 1977; Rendell and Robertson 1989, 1993; Robertson and Rendell 1990; Rendell and Verbeek 1995). Rendell and Robertson (1993) examined the cavity!/clutch size relationship in Tree Swallows (Tachycineta bicolor) via two experiments. The first determined that when given the choice between large (standard size) versus small (less than 50% of standard size) nest boxes, Tree Swallows preferred to nest in large boxes. Rendell and Robertson (1993) hypothesized that in small nest boxes, a reduced clutch size translates into reduced reproductive success. However, no significant differences in success (percent or number of young fledged) were found between large and small boxes, even when controlling for clutch size (experiment 2). Thus, the possibility that females were using box size as a predictor of future nestling conditions was rejected. However, the lack of a difference in reproductive success between large and small nest boxes may be explained by the fact that females in both large and small box types had the opportunity to optimize their clutch size to their environment. In this way, females may have selected the clutch size that was most appropriate to their environment (cavity size) and capabilities (parental quality), resulting in all females having approximately equal success, regardless of cavity size. Any reduction in reproductive success would indi-