While it won’t earn you a tick mark on your lifelist, I wanted to quickly summarize the Genetic and natural history of the Brewster’s Warbler I mentioned in tuesday’s post “So Many Stories.”

Brewster’s Warbler was first thought to be a distinct species when first documented in 1870. It was later realized to be a hybrid of Blue-winged Warbler(BW) and Golden-winged Warbler(GW). The Brewster’s is the more common of two possible hybrids that occur when these species interbreed.(The other hybrid is known as Lawrence’s Warbler) The geographic range of this phenomenon is where the more northern range of the GW overlaps with the more southern range of the BW. The ultimate result is the BW replacing the GW in this zone of overlap; a process that has been observed to take about 50 years.

Hopefully I do not over-simplify or complicate this but it is important to include a small genetics lesson to better explain what is happening.(I’ll put some references at the end of the post) Both the BW and GW have their own suite of traits that make them look the way they do. Some of those traits are dominate –as in if they are present, they can be seen and some are recessive –as in if they are present, they may be visible or could be hidden. Lets use a few traits of BW and GW as examples; the clear white throat and belly of GW and the dark eye line of BW are dominate traits. The yellow throat and belly of BW and the dark auricular(mustache) stripe of GW are recessive traits. When a BW and GW breed and have offspring, the offspring are all 1st generation Brewster’s Warblers. These birds have a mixed bag of genetic material and generally exhibit a dark eyestripe and clear white to yellowish throat and belly. It is important to note that rarely do Brewster’s males and females breed, they most often breed with a ‘pure’ GW or BW. So now if these 1st generation Brewster’s breed with either a pure GW or BW, the result is, in a perfect world, either a Brewster’s Warbler clearly exhibiting the dominant traits (as was the bird I observed) or a Lawrence’s Warbler clearly exhibiting the recessive traits. The odds are in favor of Brewster’s Warbler 15:1. There are a few complications and alternatives in this process I will avoid for now but the gist of what’s happening is hopefully clear.

An interesting aside is that the songs of these birds also become mixed and matched adding to the confusion (or excitement depending how you look at it).

Examples like this test the definition of what a “species” is and have undoubtedly led our current ecological understanding to focus on populations rather than individuals.

My main source was Peterson’s Field Guide to Warblers which probably has the single best write-up on the topic.

The Cornell Lab of Ornithology has a Golden-winged Warbler Atlasing project that is also worth checking out.

http://www.birds.cornell.edu/BOW/GOWWAR/

http://www.bsbobird.org/graphics/BRWA4.jpg