Why wouldn't the expected pattern be observed in body shape or voice? The authors suggest that because body proportions tend only to change when adapting to different ecological conditions, it is expected that they would find little change when all their sample species are in the same ecological conditions. Additionally, the authors suggest that the lack of pattern may be an artifact of their analysis, and not a legitimate result.
But what of plumage? A bird's feathers are a means of visual communication
, used to fend off rivals, attract mates, and maintain social groups. They are intimately tied to behavior, and to interactions with other individuals of the same species. What we are seeing is plumage — a trait associated with social behavior like signaling — diverging faster than traits associated with adaptation to new habitats, like body proportions. What does this suggest? I'll let Winger and Bates answer: