Birds are among the most colourful creatures on earth. But imagine being able to create a bird even brighter than nature intended. By anointing parrots with the juice of a frog, the native inhabitants of Guadeloupe in the 1700s were able to generate birds of the most brilliant hues.
Throughout history people have been fascinated by the colours of feathers and have sought ways to "improve" on nature. Now, after centuries of amateur tinkering, scientists are now able to reveal some of the underlying processes that determine the colours of birds.
The first birds to be domesticated were geese and pigeons, some 5,000 years ago. The chicken followed a few centuries later. A sure sign of domestication in any species is a change in colour, usually a loss of pigment; hence white doves and geese, but also white mice and rats.
Captive breeding – inbreeding in particular – often resulted in the spontaneous appearance of individuals with washed-out colouring. People loved the unusual and fostered such oddities. Some colour variations arose as "sports" or one-off mutations, but others appeared only gradually and had to be captured through generations of selective breeding.
The canary is a prime example. On its native Canary Islands it is a nondescript grey/green bird. Some time in the 16th century a few individuals with one or two yellow feathers (lacking the dark pigment melanin) appeared. By breeding such birds together the amount of yellow plumage gradually increased until, by around 1700, fanciers had fully yellow birds. These early stockbreeders operated with no knowledge of genetics and used only their native wit, but what they achieved was remarkable.
Bird fanciers could not resist tinkering, even with birds such as the Gouldian finch, whose natural plumage was already brilliant. Domesticated for little more than a century, the Australian finch sports a gaudy patchwork of red, yellow, purple, green, black and blue plumage. It would be hard to imagine a more colourful bird. Yet fanciers have strived to produce a white one.
The budgerigar, which is plain olive green in its native Australia, now exists in no fewer than 30 colour varieties. Another bird from the Antipodes commonly kept as a pet, the zebra finch, illustrates how far the bird breeders' genetic tinkering extends. The plumage of the male zebra finch consists of a blaze of coloured patches, stripes, spots and bands. Amateur bird-breeders have in effect dismantled the part of the finch's genome dealing with its plumage and can now create – almost to order – birds with or without any of these patches.
For species such as these, colour is entirely controlled by one of a handful of genes, so it is susceptible to genetic manipulation. The longer people have had to tinker, the more varieties they have produced: the pigeon is the record holder with more than 350 colour varieties created.
More direct and immediate intervention can produce instant changes in plumage. I remember as a schoolboy visiting the bird market in Leeds, where you could buy a tiny Indian bird, the red avadavat, dyed either a brilliant metallic blue, metallic green or yellow. The colour was short-lived; once the bird moulted (if it lived that long) it would revert to its natural reddish colour.
The practice of plunging little birds into vats of dye is now less common. Other tricks bird-dealers used were even more devious. In the 18th century, a specialist industry arose producing multicoloured parrots. This seems perverse, because parrots are among the most colourful of birds, but these psychedelic ones were extremely popular. The poor birds had their feathers pulled, and as the new feathers began to grow they were injected with coloured inks or anointed with the blood of a one particular kind of frog.
The Comte de Buffon in his Histoire Naturelle, published in the 1790s, described the frog as "azure in colour, with longitudinal bars of gold". This is the aptly named dyeing frog, Dendrobates tinctorius, one of the poison-dart frogs. Its skin contains a cocktail of alkaloid poisons. Like many alkaloids, these are mutagens. Applied to the rapidly dividing cells in a growing feather, they cause a local genetic change – and a colour change in the parrots' feathers. It was rather unpleasant for both frog and parrot; as Buffon wrote: "The plucking of the feathers hurts the birds greatly, and so many die of it, those which survive are very rare."
Dyeing birds "from within" was also popular. By the mid 1800s canary breeders had found that they could turn yellow birds orange by stuffing them full of red peppers. Fed during the moult, red peppers would produce birds the colour of marigolds. The process also worked in reverse, and birds that in the wild were naturally red, such as crossbills or flamingos, often lost their redness in captivity.
Red birds have been the focus of research into colour, because since the 1920s it has been known that red plumage is often acquired only by ingesting substances called carotenoids. Captive flamingos deprived of their natural diet of shrimps – which contain high levels of a carotenoid called canthaxanthin – turn white or pale pink after moulting in captivity. Feeding them on shrimps then restores their pinkness.
It isn't quite so easy to recapture the redness of faded cage birds such as linnets or crossbills. Research by Geoff Hill at Auburn University, Alabama and his colleagues has shown that like many other red finches, male house finches in captivity turn a pale yellow due to a lack of carotenoids.
House finches vary greatly in colour across North America; in some areas the males are naturally orange but just a few kilometres away they can be deep red. When the first settlers took red house finches from California to Hawaii in the 1870s, the birds turned orange within a few generations, causing one scientist to think that they had spontaneously mutated.
Hill recently shipped Hawaiian house finches back to the US and kept them on a diet rich in carotenoids, and when they next moulted, their redness was restored. Natural carotenoids, it appears, are scarce in Hawaii and in some other parts of North America. But there is a lot of variation even in areas where most males are red, and some males remain orange.
Only the male house finch has colour; the female is a dull brown. Male finches are red because females prefer them that way. Redness is a signal of quality, reflecting a male's ability to forage efficiently (for carotenoids, among other things) and is a sign of good health – sick birds turn yellow after their moult.
Remarkably, when female house finches from Hawaii were offered a choice, they preferred red males even though they might never have seen a red male. So, while the environment in Hawaii had robbed male house finches of their natural colour, it hadn't altered the females' preference.
Analysis of the pigments in feathers has allowed Riccardo Stradi, a biochemist at the University of Milan, to produce a carotenoid profile for each species. A few red birds, such as the European bullfinch, rarely fade in captivity, while others like the house finch invariably moult out yellow. Comparing their carotenoid profiles showed an important difference, probably regulated by only a few genes. The bullfinch has a particular carotenoid (a-doradexanthin), reflecting its ability to transform ordinary yellow carotenoids into red ones. In contrast, the house finch and all other birds that fade from red to orange in captivity lack this ability.
The carotenoids allowing birds to make yellow plumage are abundant in the seed diets of cage birds, but only a handful of species possess the biochemical pathways to transform them into red pigments for their feathers. Redness in these birds appears to reside in a combination of the right genes and the right environment.
Tim Birkhead is professor of behaviour and ecology, specialising in the study of birds, at the University of Sheffield
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