A Whiter Shade Of Pale

The week has been a frustrating one of dull days, bouts of rain and irritating wind speeds that preclude even an hour or two of ringing. Consequently I've not done a lot of anything birdy wise. 

During the dry spell of June and July the garden was devoid of birds apart from a few Goldfinch, the usual Woodpigeons and a passing Great-spotted Woodpecker. Last night when dozing half asleep I heard the screech of a Tawny Owl that sounded very close, probably in our apple tree. It’s about now that Tawny Owls start to sort out their winter territories as a prelude to their often post-Christmas breeding and each year and there’s always a pair in the trees just along the road from here. 

Tawny Owl 

But in the last few weeks, and with the change of season, lots of Goldfinch reappeared on the feeders in some numbers. These birds are about 90% juveniles and probably the second or even third brood of their productive parents. Yesterday I counted up to 20 Goldfinch at a time on the feeders, so goodness knows how many individuals that represents using the garden during the course of a day. 

So when Sue went off shopping to the big city, I was left home alone with just a mist net for company and where the breeze was not a major problem in the sheltered back garden. I caught just Goldfinches and no other species, exactly as expected where Goldfinches are by far the most common bird in our semi-rural location. 

What I didn't anticipate was to catch a very leucistic Goldfinch. It was one born this year. With its washed out appearance and lack of pigmentation I sexed it as male by a combination its long bill, the lengthy wing and healthy weight. I didn't see or catch any similar birds so there is no way of knowing if this was a one off, if there are similarly affected siblings or whether it inherited the leucism from a parent.  

Leucistic Goldfinch 

Leucistic Goldfinch

'Normal' Goldfinch

Here’s a little more about leucism & albinism in birds. From the British Trust for Ornitholgy (BTO). 

"Leucism is an abnormal plumage condition caused by a genetic mutation that prevents pigment, particularly melanin, from being properly deposited on a bird’s feathers. This results in white feathers, unless the normal plumage colour also comprises carotenoids (e.g. yellows), which remain unaffected by the condition. Although leucism is inherited, the extent and positioning of the white colouration can vary between adults and their young, and can also skip generations if leucistic genes are recessive. 

The reduction of pigment in leucistic birds causes feathers to weaken and be more prone to wear. In some situations this can hinder flight, which, in addition to leucistic birds usually being more conspicuous, can heighten risk of predation. There is also evidence that leucistic birds might, on occasion, not be recognised or accepted by a potential mate. 

Leucistic Goldfinch 

Leucism is an umbrella term to encompass a number of plumage irregularities that can be difficult to distinguish from each other. One of these is called ‘progressive greying’, which also results in white feathers. While leucism is heritable, progressive greying is not – but without knowing the history of a bird, these two conditions are difficult to tell apart. 

‘Dilution’ is another condition grouped under the category of ‘leucism’. Here, plumage colour often appears ‘washed out’ (i.e. ‘diluted’). In dilution, melanin cells are present (unlike in leucistic birds) but produce less pigment than normal. White feathers can also be caused by chromatophore (pigment cell) defects, rather than an absence of melanin-producing cells. 

Albinism also results in white feathers but true albinos are thought to be rare in the wild. Albinism is caused by a genetic mutation causing an absence of tyrosinase in pigment cells. An albino individual is unable to produce melanin pigments. This leads to a good diagnostic feature with which to distinguish leucistic and albino individuals – the colour of the eye. 

Leucistic Goldfinch 

Albinos have pink eyes while the iris pigmentation of leucistic birds remains dark. Most albino birds die soon after fledging, primarily as a consequence of their poor eyesight, and albino birds are not thought to progress to adulthood in the wild. As with leucistic individuals, albinos can retain carotenoid pigments if normally present in the plumage. A common misnomer is ‘partial albino’ – this is not possible since albinism affects the whole plumage of a bird, not just part." 

Because so many birders rely on plumage colours and patterns for bird identification, seeing an unusual bird with lighter colours or white patches can initially be confusing. By understanding what leucism is and how it can affect birds, birders can better appreciate the great variety of avian life they see. 

Linking this post to Anni's Birding Blog.