American Robin Turdus migratorius seems to be more prone than other birds to the condition known as leucism, a condition where there is a partial loss of pigmentation, resulting in white, pale or patchy colouration of the feathers, but not the eyes. I have seen leucistic robins fairly frequently and have blogged about this phenomenon before.
Recently, Mary Voisin located a leucistic bird in her neighbourhood and let us all know about it. Franc Gorenc captured these two photographs of the bird.
As you can see this bird has very substantial loss of regular colour.
Based purely on observation and without applying scientific rigour it seems to me that these birds do not seem disadvantaged in any way, as might be the case with an albino bird, for example. It begs the question, however, whether they are more visible to predators, and are thereby in greater danger of capture.
I was curious about the following bird which I saw the other day, which has two small white patches on its back.
I am assuming that is leucism to a very minor degree but it verges on serendipitous that the spots would be geometrically arranged in this way.
Leucism certainly compels us to examine American Robins closely for interesting variations in plumage.