The newly described technique of extracting corticosterone (CORT) from bird feathers may serve as a less invasive, more integrated measure of a bird's stress response. Previous work indicated that elevated plasma CORT resulted in poorer quality feathers during molt. We tested the hypothesis that a direct link exists between plasma and feather CORT concentrations. We experimentally increased plasma CORT concentrations using implants and found that the corresponding rise in CORT could be detected in feathers grown during implantation. Furthermore, CORT levels in two feathers grown at the same time from the same bird were very consistent. These results provide evidence that elevated CORT is a causative factor in decreasing feather quality during molt. However, there remain technical details that suggest caution when interpreting data from CORT extracted from feathers. Different portions of a growing feather did not necessarily reflect changes in plasma CORT at the time different parts of the feather were forming, a standard pool of homogenized feathers indicated that sample mass affects measured feather CORT concentration, and different antibodies produced different measured CORT concentrations, leaving in doubt the exact steroid being assayed.