Scarlet ibis, Eudocimus ruber (formerly, Guara rubra; protonym, Scolopax rubra), also known locally as the gaurá, photographed at Belém, at the mouth of the Amazon, Brazil.
Question: How does this elegant Brasilian mystery bird get its intense plumage colour and what is its purpose? This species also has black wingtips; what might be the evolutionary reason for that? Is this species monochromatic? Can you identify this species?
Response: This is a scarlet ibis, Eudocimus ruber. These elegant birds are monochromatic, meaning that males and females are equally colourful. (Males are somewhat larger than females.) Scarlet ibises consume a diet of frogs, reptiles and crustaceans -- and it is the carotenoids within the bodies of the crustaceans that give scarlet ibises their stunning colour. The birds concentrate the carotenoids that they obtain from their diet and deposit these pigments into their feathers when they moult. The brilliant colouring helps the adults attract mates -- the brighter their colour, the more attractive they are as mates.
There are several hypotheses that may explain this observation. Since birds with a high parasite load cannot store as much carotenoid pigments, their colouring is not as brilliant as that of healthier individuals. So brighter colour is associated with better genes.
Also keep in mind that older birds are brighter in colour than younger birds. The "handicap principle" suggests that these older, brighter-coloured birds are more clever and able to evade predators despite the handicap provided by their bright colouring, and thus, they have better genes.
Because the brightness of an individual's plumage can be used to reliably predict its overall quality as a potential mate, this is an example of an "honest signal".
Juvenile scarlet ibises resemble white ibises due to their lack of scarlet colouring, and some authorities believe the scarlet and white ibis to be subspecies.
The black wingtips are the only part of the scarlet ibis's plumage that is not scarlet in colour. This is due to the presence of the dark pigment, melanin, in the feather structure. This pigment helps strengthen the feather and provide greater resistance to abrasion, as fingsaint pointed out.