An Experimental Study of Brood Reduction and Hatching Asynchrony in Yellow Warblers

Abstract

The purpose of this study was to examine Lack's (1947, 1954) brood-reduction hypothesis concerning hatching asynchrony, and its importance in explaining the evolution of hatching asynchrony in Yellow Warblers (Dendroica petechia). The study was conducted during the summers of 1988-1990 on a population of Yellow Warblers breeding in the dune-ridge forest at Delta Marsh, Manitoba, Canada (58011'N, 98?19'W). I examined the brood-reduction hypothesis experimentally by inducing a sample of females to delay incubation until clutch completion. This was achieved by removing eggs as they were laid, and placing them in a cooler at 200C until clutch completion. As a result, eggs in each manipulated clutch hatched synchronously (hatch spread between firstand last-hatched nestling ?24 hr). Subsequently, I compared the fledging mass and survival rate of nestlings in asynchronously (unmanipulated broods, hatch spread > 24 hr) and synchronously (manipulated) hatching broods. Consistent with the hypothesis, nestling mortality was concentrated in last-hatched nestlings in asynchronous broods and random with respect to hatch order in synchronous broods. Also in agreement with the hypothesis, fledging mass and survival rates of first-hatched chicks were generally higher in asynchronous broods. However, contrary to the hypothesis, fledging mass and survival rates were similar for lasthatched chicks in asynchronous and synchronous broods. Also consistent with the hypothesis, fledging success (no. fledged/clutch) in broods of five tended to be higher in asynchronously hatched broods, regardless of food availability. Finally, fledging rate (no. fledged/no. hatched) was significantly greater in asynchronous broods (four and five nestlings combined) when compared to synchronous broods, especially when food was limiting.