A hidden cost of reproduction: the trade-off between clutch size and escape take-off speed in female zebra finches

1. The concept of reproductive costs is a central tenet of life-history theory, but the proximate mechanisms whereby such costs are mediated remain poorly understood. In this paper we demonstrate a link between clutch size and escape take-off speed in small birds, mediated through changes in flight muscle volume during laying. 2. In a series of experiments the same adult female zebra finches (Taeniopygia guttata) were made to lay both large and small clutches. When producing the larger clutches, females lost more flight muscle condition and became significantly slower to take off during escape flights. 3. This reduction in take-off speed with increased egg production was evident after the last egg was laid, and was independent of changes in body mass, so was not simply due to the presence of the eggs inside the female. 4. This indicates that there is a trade-off between fecundity and the maintenance of somatic musculature critical to an animal's escape response. As changes in take-off speed can have disproportionate effects on the chances of ground-feeding birds surviving a predatory attack, the degree of reduction in flight muscle condition during laying could be a constraint on clutch sizes in such birds.