AbstractIn order to maintain functionality of the plumage, birds apply a variety of fatty substances to the feathers, usually derived from the preen gland. Such secretions could also have cosmetic functions, altering plumage reflectance, and there is indeed some evidence for such cosmetic functions of preen fats. Some birds, noticeably pigeons, use fats derived from modified feathers, so called fat quills, in a presumably similar fashion, but almost nothing is known about their putative functions. Here we present a first test of the possibility that fat quill secretion could function as a cosmetic. As models we used domestic pigeon races that produce fat quills, we first confirm their extensive use in plumage maintenance behaviour. We then assessed the effect of experimental addition and removal fat quill secretion on reflectance of white feathers using reflectance spectrometry and physiological models of avian colour vision. Addition of secretion to cleaned white feathers resulted in a significant, discriminable change in the feather reflectance spectrum, which was strikingly similar to the absorbance spectrum of passerine preen gland secretion. However, like previous studies on preen gland secretion, removal of fats from intact feathers did not significantly alter feather reflectance, indicating that fat quill secretion when present in quantities as applied by the birds does not noticeably affect plumage reflectance. Therefore we conclude that the potential for the fat quill secretion to modify plumage colour appears limited at best, and other functions, such as antibiotic defense against feather-degrading bacteria, should be considered.