Like flamingos and spoonbills, the scarlet ibis cannot make its own pigment. It obtains carotenoids by probing mangrove mud for crabs, particularly small red crabs, along with shrimp and aquatic insects. The pigments are metabolised and deposited into feathers as they grow.
The process takes time. Juveniles are grey and brown with a white belly, and pass through a mottled stage before reaching full adult colour at around two years. Because the colour depends entirely on intake, captive birds fed without carotenoid supplements fade towards pink or white, and zoos supplement accordingly. Its close relative the American white ibis, with which it interbreeds where ranges meet, takes broadly similar prey without turning red.
Why the two differ so sharply has not been fully worked out, and the taxonomic question of whether scarlet and white ibis are one species or two remains unsettled.