Effects of supplemental calcium on the growth rate of an insectivorous bird, the purple martin (Progne subis)

Abstract

The calcium content of insects is postulated to be insufficient to meet the high demands of skeletal growth incurred by nestling insectivorous birds. To compensate for this calcium deficiency, adult birds commonly supplement the diet of their nestlings with objects rich in calcium, such as mollusk shells. This leads to the hypothesis that nestlings of species that have difficulty obtaining calcium-rich objects may suffer from calcium-limited growth rates. In this study, we experimentally tested whether supplemental calcium affected the growth rate of an aerial insectivore species with limited terrestrial mobility. We force-fed nestling purple martins (Progne subis) a calcium supplement and compared their growth rate to a control group. Calcium supplementation did not affect the growth rate of nestlings as measured by mass, primary feather length, tail feather length, wing bone length or tarsus length. The lack of effect was likely a result of martins feeding nestlings calcium-rich objects. We conclude that despite the terrestrial agility problems suspected to hinder aerial insectivores, they are capable of collecting calcium-rich objects, and that the growth rate of their young is not limited by a calcium-deficient diet.