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Several characteristics of plumage, including color and molt schedule, are influenced by hormonal signals, and hence may be modified by endocrine active contaminants. If so, the functions of plumage (e.g. communication for mating or territorial defense), may be compromised by exposure to such compounds. Polychlorinated biphenyls (PCBs) are ubiquitous and persistent environmental toxins that can disrupt endocrine function in laboratory animals. Captive American kestrels (Falco sparverius) were fed PCBs (Aroclor 1242) at 6 and 60 ppm, thyroxine at 10 ppm, estradiol at 1.4 ppm, and a thyroid hormone blocker (propylthiouracil: PTU) at 2000 ppm, mixed in their normal diet. Plumage characteristics measured included: the width of the black subterminal band on the tail, brightness (a composite index of hue and saturation), reflectance from 230 - 800 nm (measured on a reflectance spectrophotometer), the pattern of feather loss and regrowth on the tail and wing, and the timing of onset and the duration of molt. PCB-treated birds did not differ from controls, but birds treated with thyroxine were significantly different from those dosed with estradiol or PTU in the width of subterminal tail bands, reflectance, and duration of molt. Thus, although hormone treatments did modify some of the plumage characteristics we measured, PCB treatments at these exposure levels did not.