Flamingos feed with the head upside down, drawing water through a bill lined with hair-like lamellae and using the tongue as a pump. What they strain out includes cyanobacteria, diatoms and small crustaceans such as brine shrimp, all rich in carotenoids, particularly canthaxanthin and related compounds.
Birds cannot synthesise carotenoids. The pigments are absorbed in the gut, transported by the liver, and laid into feathers as they grow, as well as into bare skin and the bill. Diet therefore controls colour directly: captive flamingos fed without pigment supplements fade to white, and zoos add carotenoids to keep birds pink. Colour intensity varies between species and populations chiefly because their food differs, with the lesser flamingo, which eats pigment-dense cyanobacteria, tending to be deeper in colour.
One subtlety often lost is that colour is not merely cosmetic. Brighter birds display more and pair earlier, and adults deplete their own pigment while raising chicks, so a faded parent may be a well-fed one.