AN EXPERIMENTAL-STUDY OF BROOD REDUCTION AND HATCHING ASYNCHRONY IN YELLOW WARBLERS

The purpose of this study was to examine Lack's (1947, 1954) brood-red uction hypothesis concerning hatching asynchrony, and its importance i n explaining the evolution of hatching asynchrony in Yellow Warblers ( Dendroica petechia). The study was conducted during the summers of 198 8-1990 on a population of Yellow Warblers breeding in the dune-ridge f orest at Delta Marsh, Manitoba, Canada (58-degrees-11'N, 98-degrees-19 'W). I examined the brood-reduction hypothesis experimentally by induc ing a sample of females to delay incubation until clutch completion. T his was achieved by removing eggs as they were laid, and placing them in a cooler at 20-degrees-C until clutch completion. As a result, eggs in each manipulated clutch hatched synchronously (hatch spread betwee n first- and last-hatched nestling less-than-or-equal-to 24 hr). Subse quently, I compared the fledging mass and survival rate of nestlings i n asynchronously (unmanipulated broods, hatch spread >24 hr) and synch ronously (manipulated) hatching broods. Consistent with the hypothesis , nestling mortality was concentrated in last-hatched nestlings in asy nchronous broods and random with respect to hatch order in synchronous broods. Also in agreement with the hypothesis, fledging mass and surv ival rates of first-hatched chicks were generally higher in asynchrono us broods. However, contrary to the hypothesis, fledging mass and surv ival rates were similar for last-hatched chicks in asynchronous and sy nchronous broods. Also consistent with the hypothesis, fledging succes s (no. fledged/clutch) in broods of five tended to be higher in asynch ronously hatched broods, regardless of food availability. Finally, fle dging rate (no. fledged/no. hatched) was significantly greater in asyn chronous broods (four and five nestlings combined) when compared to sy nchronous broods, especially when food was limiting.