They are the bizarre-looking birds which defy expectations, from blackbirds which are brilliant white, to shocking pink house sparrows.
Now, following an increasing number of reports of birds with the "wrong" coloured feathers, ornithologists have launched a study to investigate what could be behind the strange sightings.
The phenomenon affects many of the most common species, including several known for their distinctive colourings, such as red kites and robins, both of which have been observed without their renowned red plumage.
As well as all-white blackbirds, there have also been reports of coots, crows and moorhens with all white rather than black feathers.
Other cases include black-headed blue tits and yellow rather than red woodpeckers.
Experts believe the discolouration is often associated with genetic conditions but say that plumage can also be affected by environment and diet.
House sparrows living near a salmon farm at Kinlochbervie, in the Scottish Highlands, have developed a distinctly pinkish tinge after feeding on fish pellets containing prawn shell, designed to give the salmon flesh a pink colour, while the yellow colouration on blue tits relates directly to the amount of caterpillars in their diet. So a shortage can lead to this colour fading.
The study, by the British Trust for Ornithology, will look for other similar cases as well as investigate the behaviour of affected birds, which often act differently to those with more traditional appearances.
Robins with white breasts appear to be more shy and cautious than those with red breasts, the vibrancy of which are seen in the species as a symbol of dominance.
In some cases, strangely-coloured birds are not recognised by potential mates and their unusual plumage can mean they stand out more to predators.
As part of the study, which is the first of its kind, members of the public are asked to register all sightings and any observations about behaviour.
The sightings are being compiled in a database and will be analysed alongside existing statistics about bird populations, to help identify trends.
From these, scientists will be able to establish if there are particular "hot spot" parts of the country and whether certain species have higher "abnormality rates" and will allow for further investigation into what is behind the phenomenon.
The BTO first appealed to members to send in their sightings less than a month ago, and have already received more than 700 reports. Initial indications are that the species most commonly affected are sparrows, blackbirds and chaffinches.
The most common variation of discolouration is thought to be leucism, a genetic condition where one of more pigments are lost, producing "snowy-looking" birds.
Leucistic birds can appear all white but retain normal coloured eyes unlike true albinos whose eyes are pink. It can also be found in other animals and humans.
Dr Tim Harrison said: "We get a substantial amount of reports about these birds so now we want to investigate more. This study will allow us to ask further questions about why certain birds in certain areas are affected and what is behind it.
"These birds are inherently interesting because of their appearance and we are also interested in the way in which they interact with others."
To take part in the Abnormal Plumage Survey online, visitwww.bto.org/gbw or telephone 01842-750050 for a paper recording form.
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