A strong case of dilution in the Yellow-throated Toucan (Ramphastos ambiguus)

Abstract

Plumage color aberrations are common in birds, but often it is difficult or even impossible to identify them properly in the field. Several of these aberrations are common, especially progressive greying, leucism and ino, although there is confusion among the different mechanisms. Other aberrations are rare or infrequently reported. Dilution, for example, refers to a reduced concentration of melanin granules that dilutes the color, although the pigment itself is not changed. It affects the entire plumage, rather than single feathers. The Yellow-throated Toucan (Ramphastos ambiguus) is predominantly black, and has feathers with brown tips, red under the tail coverts, and feathers above the tail are cream colored. It has a yellow bib covering the throat and chest with red band countering it, and the skin of the face is chartreuse. The Yellow-throated Toucan is most notable for its massive, bicolored bill. Here we report an apparent strong case of dilution in the Yellow-throated Toucan. An individual observed at La Unión, Guápiles on the Caribbean versant of Costa Rica on 10 October 2019 almost lacked melanin, but retained carotenoids. The tertials and some feathers on the rump were very light grey, suggesting strong dilution. We also discuss other possible explanations for this amazing coloration.