Thursday, February 23, 2012

Mutant Crows




































We have a weeping pear (Pyrus salicifolia) tree in our garden which is regularly targeted by crows for materials for nest building and nest repair at this time of year. Its slender  twigs are long and pliable - desirable qualities for weaving into a nest - and the birds struggle mightily to tear them off, often with limited success. This morning the tree's assailant was a little different - a mutant crow with white wing primary and covert feathers.





This individual was due for a moult, judging by the wear and tear visible on the primary feathers, which are very tattered.


Single birds with groups of unpigmented feathers like this turn up quite frequently in most species (including our garden blackbirds) but what was interesting about this morning's visitor is that it was one of a pair, with virtually identical mutant markings. 


A single crow with some white feathers that mates with a normal all-black crow would be likely to produce all-black offspring if the white feathers were due to recessive gene mutation but if both birds in the pair were similarly mutant then some or perhaps all offspring (depending on how may genes are involved and how they interact) would inherit the trait, so an increasing number of birds like this should appear in the population. The fact that these two mutants seem to have paired up raises some interesting questions about how they choose potential mates. Each would have had an unlimited supply of normal all-black mates to choose from but they see to have chosen a mate that resembles themselves. This kind of non-random mating in the population (assortative mating, in the parlance of evolutionary biologists who study population genetics) should increase the frequency of their particular feather patterns in the population. It'll be interesting to see if white-winged crows become more prevalent around here in future years if these two are indeed a breeding pair and do successfully raise a brood that are similarly marked and show a similar preference for choosing mates.



14 comments:

  1. Fascinating, I hope you'll let us know if you start seeing more of these. Never seen white winged crows over here. However, I've been so lucky lately to have seen a white red-tail hawk several times on my way to work. Amazing bird! Very much like a gyrfalcon, but with the characteristics of the somewhat bulkier red-tail.

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  2. As the years go by I seem to see more birds of various types with a few white feathers. Not seen any Crows like that though.

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  3. Hi Phil, although genetically inherited partial albinism is likely in other species, carrion crows are affected by feather discoloration that has been attributed to some mineral/vitamin deficiency, it is very common in the population in East Park in Hull:
    “Partly coloured feathers are very unusual in leucism. Individual feathers that are partly coloured usually indicate a bad condition of the bird during feather growth and is not an inheritable character (ie, is not leucism). This is often seen in, eg, Carrion Crows, especially those eating junk food in cities
    You can find more info here (where I got the excerpt):
    http://www.housesparrow.eu/pdfs/vanGrouwHein2006_NotEveryWhiteBirdIsAnAlbinoSenseAndNonsenseAboutColourAberrationsInBirds.pdf

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  4. Hi Valerianna, I really envy you your encounter with the red-tailed hawk - sightings of raptors are always a thrill!

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  5. Hi John, Looks like it might all be down to the poor diet in the North East (see Africa's comment)...

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  6. Thanks Africa, I'd completely overlooked the dietary dimension .... I should have remembered, as a while ago I found old references to the effect of diet on bullfinch plumage. I guess the fact that the two looked so similar led me to jump to conclusions but when you look at the poor condition of the birds' plumage diet is certainly the more plausible reason. They must be living in the bins of the local fast-food outlets!

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  7. I agree, no matter how many times I see raptors, I am entralled. In the forest here, I see quite a few. However, seeing a WHITE one shinging in the morning light from a very tall highway lamppost was really something. And that I have gotten to see it more than once is really special.

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  8. What an extraordinary post with Africa making a very good comment too. It always pleases me to learn new things, so thank you both.

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  9. I wonder if there is a reason why the crow family and thrush family seem to throw up more mutants than others (or at least that seems to be my experience).

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  10. Crows with white in the wing are increasing along the coast here in South Northumberland.I suspect they do exist on a poor diet of discarded fast food mainly chips & batter,I had thought it was an extended family group !
    Brian

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  11. Mine too John, although I guess it's likely to be more conspicuous in anything with black plumage. I did once photograph a chaffinch that had a lot of white feathers...

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  12. Hi Brian, I'm amazed that those eiders in Seahouses harbour don't have plumage abnormalities too, given the amount of chips that are fed to them!

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  13. Hmm.... the crows around Norton green nearly all have a good few white feathers... and a few down the high st have, but not all.... but not so heading into stockton, where you would expect to be more junk food... but it is an interesting point. I thought i'd found a rare extended family group too... ah well...

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  14. Hi Suzi, I wonder why city pigeons aren't noticably more prone to white fathers than other birds - they muct have the ultimate junk food avain diet....

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