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Among the many challenges faced by a beginning birder is how to handle birds that do not look quite right. Most birds look more or less like the pictures in the field guides; others show a range of variation, but are close enough to identify with a minimum of difficulty. Yet others can differ quite substantially, particularly in plumage. In some cases this is due to albinism or leucism, variations in which a bird is extraordinary white-colored. In other cases, melanism - extreme darkness - is the cause. Hybridization is another frequent cause of variation.
Some bird groups are more prone to these than others. Rock pigeons, for one, are well-known for occurring in many different color morphs, probably thanks to selective breeding of domesticated pigeons. Waterfowl, especially the common mallards, are also highly variable.
On a recent walk I ran into this odd pair.
The two ducks pictured above are both mallards, a male and a female. They seemed to be a pair since they stuck close by each other for the whole time I observed them. I have seen these two before at the same location, and each time they appeared to be together.
What appears so unusual about these two is that the female is extremely light, and the male is extremely dark. Unfortunately, because the light was not ideal, the contrast does not come through completely in these photographs.
While most female mallards are a medium-brown, this bird was a very light brown, almost sandy-colored. The contrast between light and dark patches on her body was much stronger than on a normal mallard.
Her apparent consort was, in turn, far darker than normal male mallards. The head was dark green, as on most males. His breast was duller than the chestnut that characterizes mallards. But the contrast was most noticeable on the wings and flanks. These parts on a normal male are generally pale to medium gray. On this bird, both wings and flanks were a very dark gray-brown. 
Both of these birds are clearly mallards, but appear much different from other members of their species. So, how does one make this identification with confidence? In a case like this, shape and behavior hold the key. These two birds were associating closely with a large flock of mallards, and engaging in the same sorts of activities as the rest: combing through the grass while on the shore, and dabbling for food while in the water. The close association with mallards allowed me to compare the relative size and bill shape with the other mallards in the area. All shape and size characteristics matched well. These are not the best photographs, but even from these, you can see that the underlying plumage patterns match those of mallards.
I think that these two are not hybrids. Usually hybrids show elements of both species involved. The most common mallard hybrid is a cross with a black duck. Some keys for identifying this cross are found here. Photographic examples can be found here.
Cross-posted at Blue Ridge Gazette.
