Nestling Eastern Kingbird Growth: Effects of Initial Size and Ambient Temperature

Abstract

Growth, defined in terms of mass gain and increase in lengths of tarsus and ninth primary wing feature, was studied in nesting Eastern Kingbirds in relation to a suite of environmental variables. These data indicate that many factors influence survival probabilities and final size attained by nestlings, though initial mass (egg size) seems of primary importance. The impact of weather variables was, however, nearly as important. Mass gain throughout the first half of the nestling period depended most on initial mass on the day of hatching. During the latter half of the nestling period, variation in ambient air temperature was the variable best able to explain mass variation. Nestlings lost mass on days with high air temperatures. Nestling mortality due to heat stress was also high on hot days, but only for nestlings with poorly developed powers of thermoregulation, i.e., those <10 d of age. Tarsal and primary lengths at fledging were directly correlated with hatching tarsal length and mass at day 14. Primary growth was also directly correlated with ambient air temperature.