Weekly Reports

Last week I got a very interesting call. It was the Information Officer, Michelle, from the Boreal Centre calling to tell me that a man was there who wanted help identifying a picture of what he believed to be a bay-breasted warbler hybrid. Naturally I was skeptical but curious. Hybrids are very rare in the bird world. There are a lot of factors working against their existence. For starters, female birds are attracted to a particular mate by its song – a female bird will only respond to the song of her species’ male. Then, once attracted to check out the male, she will evaluate his appearance and decide if he looks like a suitable mate. Even if the female bird responds to the wrong song and fails to recognize the male as the wrong species chances are good that he won’t acknowledge her since she is not what he is instinctually programmed to look for. If the two confused love-birds make it past all that, they face the challenge of actually creating viable eggs. Finally the young hybrids have to grow and survive migration. And this is the only way a hybrid can be made; in birds, male hybrids are fertile, but females are not (mammals are the opposite), so a hybrid cannot breed with another hybrid to perpetuate its kind. I’m not statistician, but those can’t be good odds. Then throw in someone actually seeing this bird… Most hybrid reports are not hybrids at all. There are a few situations that can cause a bird to not look how it is supposed to. In birds that have an alternate moult (i.e. specific breeding plumage); young birds often don’t have as bright or well-defined alternates, and if a bird comes under stress while undergoing its alternate it may only get half-way through it giving the bird a rough, unfinished look. Birds can also be what’s called leucistic, which is when they lack black pigmentation. Leucisism is often mistaken for albinism which is a lack of all colour pigmentation (albinos always have pink eyes as a result of this; any animal that is ‘albino’ without pink eyes is actually leucistic). The opposite of leucistic is melanistic which is having too much black pigmentation. Either of those pigment anomalies can cause a bird to not look right. So imagine my excitement when I arrive at the BCBC and the pictures are not only close and in-focus (aren’t all pictures of rarities supposed to be blurry?), but they are of what appears to be a bay-breasted warbler and blackburnian warbler hybrid. The bay-breasted half of him was easy to see – he sang like a one and had an obvious bay coloured breast but the other parent was harder to figure out. My first impression was that it was a black-and-white warbler based on the heavy white eyebrow and white stripes on a black back. Upon further inspection I believe that the other half of it is actually a blackburnian warbler. Excluding colouration, the facial pattern is an exact match to that of a blackburnian and although it is hard to see in the picture there is also has a small coloured marking on the crown of its head. Many warblers have stripes down the crown of their head or concealed crown patches, but a small coloured spot on the top of the head is distinctive of the blackburnian. The stongest evidence I have to support my theory of its parentage, however, is its wingbars. Only two species of warbler have such wide wingbars that converge together in a white patch: the blackburnian and the magnolia and considering it has no other characteristics similar to a magnolia I am left with the blackburnian. These two species do have similar songs and live in similar habitat so it is not surprising that they have been reported to hybridize before, but this is still a truly rare sighting. The picture was taken by a man named Istvan Orosi, an avid bird photographer; he is planning to make sure it is properly reported so it can be confirmed.