To support the ossification of fetal and neonatal bones, reproductive females must transfer calcium to their offspring. The extent to which calcium is mobilized from maternal bone is related inversely to the amount of calcium ingested. Insectivorous bats consume a low-calcium diet, and thus, a reproductive female may experience a conflict over allocating dietary calcium to self-maintenance or her developing offspring. We tested the ability of big brown bats (Eptesicus fuscus) to use elevated concentrations of dietary calcium to determine if available dietary calcium limits the amount of calcium that mothers transfer to their offspring. During late pregnancy and early lactation captive bats were fed diets with calcium content 10 times greater than (high-calcium) or equivalent to (low-calcium) diets consumed by free-ranging individuals. We measured the calcium content of guano produced by mothers as an indicator of calcium absorption and the calcium content of their young as a measure of total calcium allocated to offspring development. Females on the high-calcium diet defecated 13 times more calcium than females on the low-calcium diet, and no difference in body concentrations of calcium existed between pups produced by females on low- and high-calcium diets. This suggests that limited calcium intake does not constrain calcium transfer to offspring in big brown bats. It is likely that the calcium demands of offspring production are met largely by mobilizing maternal skeletal calcium.