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 Glowing parrots
Birds are highly visual animals, as their brightly coloured plumages would suggest. The means of colour production are much the same in all animal groups - birds just take it to unrivalled heights. The colours of the feathers result from two major sources: structure and pigment, or an interplay of the two. Structural colours owe their existence to the way light is reflected from the surface layers of the feather. The most common examples of this are iridescence, such as in the feathers of the peacock and birds of paradise, and the colour blue ('blue' feathers are really brown). Pigments are responsible for the other colours of feathers. The important pigments are melanins (blacks, greys, browns and some oranges), carotenoids (yellows, oranges and reds) and, less frequently, porphyrins (usually warm brown but rarely green or magenta). In most green feathers, however, the colour comes from the combination of yellow pigment with a structural blue. There is another pigment (or pigments) that have been reported only in parrots. These, rather than carotenoids, produce the yellows, oranges and reds. There are two types of yellow, one of which has a fascinating property: it fluoresces under ultraviolet (UV) or black light. Although originally reported in 1937 (O. Völker, J. Ornithol. 85: 136-146), little work has been done on this unusual property since. This phenomenon can be seen in the accompanying 'before and after' photographs. One image shows the parrot under normal lighting, the other under UV. The UV photos were taken using a commercial UV light tube and a wooden box, with the room lights turned off. Some species of parrots react strongly from many parts of the body; others fluoresce only from restricted areas; and some showed no evidence of the special pigment at all. Cockatoos and most Australian rosellas and grass parrots fluoresce strikingly, but lorikeets are be UV inert. Certain conditions are apparently necessary to obtain the desired reaction. First, the parrot must have yellow in its plumage or at least mixed in green feathers. Thus the plumage of the pink and grey Galah (Cacatua rosei - capilla) does not fluoresce under black light. Second, the yellow has to be the correct yellow. A number of parrots have only the non-fluorescing yellow or have both yellows distributed on different parts of the body. The biological significance of this is still uncertain. It may be that fluorescence is no more than a property of this particular pigment and it may not 'represent' anything to the parrots; however, the position on the body of the fluorescing sections suggests that these may have a function. Species such as the Eastern Rosella (Platycercus eximius) and Hooded Parrot (Psephotus dissimilis) glow on many parts of the body; in other species the pigment was restricted to those areas often associated with social or courtship displays (foreheads, napes, crests, cheeks and shoulder stripes). The crest of a Sulphur-crested Cockatoo (Cacatua galerita) fluoresces brightly, but is outdone by the forehead of the Budgerigar (Melopsittacus undulatus), which looks like the glowing end of a torch. Interestingly, a yellow mutation Budgerigar fluoresces in the same regions as wild type green birds - the two types of yellow pigment are quite evident. Perhaps the most intriguing species tested is the Golden Conure (Guaruba guarouba) of South America. Except for some dark green feathers on the wing, the entire bird is a uniform golden yellow. Yet, when it is placed under the UV, a bright square patch appears on its nape, contrasting with the non-reacting yellow feathers on the rest of its body. There are obviously many unanswered questions about avian fluorescence. Attention to the biochemistry of this yellow pigment will fill some gaps in our knowledge. Of particular interest is the significance of the possible signals to some species of parrot. Behavioural studies may show that 'glowing' body parts are important in the day-to-day interactions of these birds. Many birds are brightly coloured. Parrots are prominent among these, but one of their pigments has a special character - it glows in ultraviolet light.
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