Blue-winged vs. Golden-winged warblers: An interesting conundrum

 

A male Blue-winged Warbler along the Black River in Cheboygan County, Michigan on May 19, 2021. I heard the bird singing, and eventually managed a documentary shot on this dark, rainy day.

I photographed this Blue-winged Warbler in the Pigeon River Country State Forest, which is in the northern tip of Michigan's Lower Peninsula. While the species is old hat to those who live in the eastern U.S., south of Michigan, it's - for now - a rarity up here. In a dozen nearly consecutive years (missed last year - COVID) of intense natural history exploration up here, it is the first Blue-winged Warbler I have documented.

I'm not just missing them. The northern Lower Peninsula is at the northern limits - for now - of the southerly Blue-winged Warbler's distribution. eBird records remain sparse here, and most of them come from the last decade. Records will continue to increase, I am sure, and I am reasonably certain this will not be the last Blue-wing I encounter in this region.

While leading a group a few days after finding the Blue-wing, and only a few miles way from that spot, we encountered the bird above. It is a hybrid between the Blue-winged and Golden-winged warblers, and this hybrid form, which expresses dominant genetic traits of both species, is known as the Brewster's Warbler (At least for now. There is a movement, misguided in my opinion, to do away with all eponymous bird names). I revisited this spot on May 28 and made this shot. The bird was singing a more or less Golden-winged Warbler song, although it frequently elided the last syllable. Lawrence's Warbler is the other typical hybrid form but it is far scarcer. I've only seen it once, long ago. This absolutely stunning form - lemon-yellow below, bluish-gray above, with chickadee-like black bib, and prominent ebony eyeline, apparently results mostly from pairings of second generation backcrosses. Brewster's hybrids express mostly dominant genetic traits of the parent species; Lawrence's expresses recessive traits.

Golden-winged Warblers are reasonably common up here. To show how easy to find they can be, after working with the aforementioned Brewster's Warbler, I drove a few miles north to an aspen-dominated reverting clearcut and found and photographed this apparently "pure" Golden-wing for a hot off the press comparison.

PHOTOGRAPHIC NOTE: I love trying to shoot "creative blur" songbird photos. This fellow sat on this Bigtooth Aspen branch for some time, singing and preening. When songbirds preen, they often interject short violent shakes of their wings, tail, and body. The photographer should be alert for this behavior. As long as the head and eye are in focus, the shake shot will work, and create an interesting pose. Badly done, "creative blur" is just synonymous for a poor photograph. Because of the ample sunlight streaming in over my shoulder, I was stopped down to f/8. Even though my shutter speed was 1/1000 (ISO 320), it still wasn't nearly fast enough to freeze the action and we see the wings and tail in an interesting blurred fluff. Had I been more on my game, I would have probably had the shutter speed down to 1/200, though.

I "fear" I am bearing witness to the beginning of the end of the Golden-winged Warbler in the northern Lower Peninsula. I don't really fear this, though, and am rather more fascinated by the biological process of genetic swamping by the Blue-winged Warbler. When these two "species" come into contact, the process of hybridization commences, with the upshot being that within a few decades all of the Golden-winged Warblers will be replaced by genetically dominant Blue-winged Warblers.

The likely explanation for the prolific hybridization is that the two entities have only recently come into contact with one another. As European settlers opened up the vast eastern deciduous forest, the clearing of the landscape abetted the northern expansion of the more southerly Blue-winged Warbler. As it increasingly came into contact with the northerly breeding Golden-wing, hybridization occurred resulting in the genetic swamping that seems to be increasingly common. In my state of Ohio, the Golden-wings were largely vanquished by the 1940's. About 75-80 years later, I am witnessing this same phenomenon nearly 300 miles to the north of Ohio's Oak Openings region near Toledo, where Ohio's most robust population of Golden-wings was historically documented.

Sharp students of natural history might ask "If they hybridize and produce fertile offspring, why are Blue-winged and Golden-winged warblers separate species?" 

Great question.

"The Biological Species Concept defines a species taxon as a group of organisms that can successfully interbreed and produce fertile offspring. According to that concept, a species' integrity is maintained by interbreeding within a species as well as by reproductive barriers between organisms in different species." (lifted from www.sciencedirect.com).

The BSC is the most common concept that defines species, although genetic analysis and the resultant data increasing play a role in drawing species lines. Genetic studies of these two warblers also do not bolster the case for separate species. They are 99.97% genetically similar. But morphology has apparently won the day, and the two are retained as separate species based primarily on their very different appearances.

WAY back in 1835, John James Audubon speculated that Blue-winged and Golden-winged warblers were the same species. Indeed, it seems that they should be considered forms or subspecies of the same entity. It's possible a bit of politics enters into the decision to retain them as separate species. Can you imagine the hue and cry in the birding community if these two "species" were lumped into one?

Independent of the human obsession with sorting and pigeonholing organisms, and the problems sometimes encountered when doing so, I will continue to enjoy the unfolding drama of the Blue-winged/Golden-winged clash in northern Michigan. Personally, I do not see this as a conservation issue - certainly nothing approaching the true conservation concerns surrounding the nearby breeding Kirtland's Warbler. Rather it is a case of long-term (or maybe short-term!) evolution playing out its hand. The Blue-winged form holds primacy over the Golden-winged form, and who are we to say that this is bad? I suspect Nature knows better than we as to what this "species" needs to expand and flourish.