Just found it! Did not know that, learn some useless piece of info every day...
"Anyway, the flamingo's feathers turn pink as the bird matures. This is caused by chemicals in the shrimp and plankton which they ingest. If you wonder why we humans don't turn pink, it's because we are, for the most part, featherless (also we don't eat as much plankton). On your next trip to the zoo, if you look more carefully, you'll notice that most of the flamingos are large and brilliantly colored. A substantial number of them, though, are smaller and with much duller coloration. While ornithologists label the latter birds as a separate species, called by the common name Lesser Flamingo, we scientists at On Stagnant Pond are pretty sure that the ugly birds just got hold of some bad shrimp."