Abstract
Up to two extra chicks were added to a Redbreasted Sparrowhawk Accipiter rufiventris nest to determine whether the parents would increase their biomass provisioning rate. During 46 days of manipulations (day 19 to day 64) no response occurred in either of the adults' rate, or that combined. Provisioning mainly small passerines, the adults provided between 43 and 122 g d-' during the first 64 days. The adult male which remained provisioning, weaned the one remaining fledgling until the 76th day following hatch when no more prey was delivered. The male provisioned significantly (p < 1 x 10-6) lighter prey than his mate but provided about 3.67 kg of prey in contrast to the female's 1.95 kg during the hatch-to-independence period. The rate of provisioning was significantly (p < 0.05) lowered during steady or torrential rain, but temperature extremes did not affect the rate. Since available daylight hours, temperature, instrinsic prey size, and rain could not account for the smaller amount of biomass delivered (to two nests), relative to temperate breeding sparrowhawks, I suggest in conjunction with other comparisons that some subtropical raptors may be reproductively limited by food abundance.
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