Birds with poor digestion are literally off colour

Mr and Mrs House-Finch THE vibrant hues of beautiful plumage are often borrowed. Flamingos, for example, owe their pinkness to chemicals called carotenoids that are made by bacteria known (confusingly) as blue-green algae. The birds, when feeding, both ingest these bacteria directly and consume small crustaceans that themselves subsist on such bacteria. Blue-footed boobies obtain their eponymous colour similarly, via the fish they eat. Carotenoids, though, are dual-use molecules. Besides being p...