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.