Birds with poor digestion are literally off colour

Nov 23, 2017

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 pigments, they also help to stimulate the immune system. If a bird is troubled by parasites or pathogens its immune system will thus use up some of its carotenoid stock defending against these interlopers, and its colour will suffer. If it is parasite-free, by contrast, most of the carotenoids it consumes will be used to create colour. This is a difference that potential mates notice and act on, as dozens of experiments have proved. But a study just published in...

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