Why are flamingos pink?

The colour is not in the bird. It is in the lunch. Flamingos feed by standing in shallow water and filtering out algae and small crustaceans with their oddly upside-down beaks, and those foods are full of pigments from the same family as the one that makes carrots orange.

The pigments pass through the liver, into the blood, and are deposited in new feathers as they grow. The Smithsonian's National Zoo notes that chicks hatch covered in white down and take several years and several moults to become fully pink. A flamingo's colour is a slow record of everything it has eaten.

Zoos learned this the hard way. Early captive flamingos fed on ordinary bird food turned pale, and keepers now add pigment to the diet to keep the birds looking like themselves. Flamingo parents even feed their chicks a bright red milk-like liquid from their throats, and go visibly paler while raising a chick. They are, quite literally, giving their colour away.