 Ornithologists James Hancock and Jim <unk> also consider the two to be a single species , with the differences in plumage , size , skin coloration and degree of bill darkening during breeding season forming the diagnostic characters . They have proposed the populations <unk> in northwestern South America after a period of separation , and that the color difference is likely due to the presence of an enzyme that allows uptake of pigment in the diet . They have questioned whether white @-@ plumaged birds of South America are in fact part of the ruber rather than the albus taxon , and acknowledge that more investigation is needed to determine this . 
