Temporal Components of Reproductive Variability in Eastern Kingbirds (Tyrannus Tyrannus)

The influences of weather and food supply on Eastern Kingbird (Tyrannus tyrannus) reproduction were examined using data on annual (1980-1983) and seasonal variation in timing of breeding, clutch size, and egg mass in a population of kingbirds breeding in eastern Kansas, USA. Timing of breeding differed significantly among years and appeared to be related to insect abundance. Clutch size declined significantly with breeding date in all years, but differed significantly only between 1980 and 1981. A severe drought in 1980 likely depressed food availability, whereas an emergence of periodical cicadas in 1981 provided a superabundant food source. Across all years, however, clutch size varied inversely with the mean breeding date of the population, whereas average egg mass varied directly with food availability. Maximum annual differences in mean clutch and egg size were small compared to variation in food availability. I examined 10 hypotheses that have been proposed to explain seasonal changes of clutch size in birds. All but three were rejected as possible explanations for kingbirds. The tendency of older females to lay both earlier and larger clutches is probably responsible for most of the seasonal decline of clutch size, but reduced probability of survival for offspring fledged late in the breeding season and avoidance of stress in adults just prior to migration may also be contributing factors. Reduced clutch size late in the breeding period should permit females to hatch young sooner, to raise them faster, and to fledge them at higher masses. These features should increase survival of both juveniles and adults since postfledging care is long in kingbirds.