While off work I have checked on a few ponds that still have cygnets from last year. They need to move on as it won’t be long before we have the class of 2019!
At Barnes’ Pond I got chatting to someone who questioned why one cygnet was already completely white. I said that it was a Polish Swan and we had quite a lengthy chat about these unusual swans. The woman was surprised by what I said and it made me decide to post something about Polish Swans.
Science was never my strong subject so I am going to avoid explaining how Polish Swans occur. Just to say they are not albino and more akin to a leucitic water bird. For those interested I have copied the BTO’s description below which explains Leucism and Albinism a lot better than I could. If you’re not interested just skip this bit!
Leucism & albinism
In leucistic birds, affected plumage lacks melanin pigment due to the cells responsible for melanin production being absent. This results in a 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.
In our Abnormal Plumage Survey, ‘leucism’ is being used as 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 that we have grouped under the category ‘leucism’ in our Abnormal Plumage Survey. Here, plumage colour often appears ‘washed out’ (i.e. ‘diluted’). This Collared Dove (pictured) is a good example of this. 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.
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.
Anyway, that’s the technical stuff out of the way but I do know that if you see a Polish Swan it’s much more likely to be a female. Males are usually carriers of the gene but having said that there are still some male Polish Swans. I do know that if both parents are Polish then all their cygnets will be Polish too but I’m yet to see a pair of Polish swans.
They were first described over 170 years ago when they were sighted around Poland – hence their name. At the time, they were thought to be a totally different species to the Mute Swan. There has been very little research done into these swans – specifically their distribution around the United Kingdom – until a paper was published in January 2018 following a survey that was carried out in 2016. Despite being rare we have two Polish families within a couple of miles of each other. I am convinced that both of the breeding females are related but, of course, it’s impossible to prove. They have BTO rings but they were not added until both swans were adults. I do know that pens often return close to where they were born, when they breed.
Polish Cygnets are born white and look very different from their grey siblings and yet very few people seem to notice this when they are feeding them. They have much paler legs and once they become adult this is pretty much the only way to identify them. In my experience, and that of other rescuers, they seem less robust than other cygnets.
The breeding pair of swans at Barnes Pond produce some Polish cygnets each year although the majority of the brood are grey. However, the other pair, that breed in nearby Roehampton, haven’t had Polish cygnets for the last couple of years. Although they have moved locations, we know they are the same pair as their BTO rings are checked annually. There’s no doubt Polish swans are very rare but as most swans spend the majority of time in water the number could actually be under reported. They are virtually impossible to identify unless their legs are visible. Until 2013, the numbers logged were in single figures. Even the highest figures, logged in 2016, show only 37 Polish cygnets to have hatched in the UK. I hope future swan surveys describe Polish swans and encourage members of the public to identify them separately. We might find that there are more Polish swans than we think.