Tyrant Flycatchers and Evolution

Science places all living organisms in groups. Humans and birds are part of a phylum whose name is chordata, because all in the group have an internal skeleton rod. The next grouping is called the class, and the class to which birds belong is aves. Within aves are two dozen "orders," and within orders are "families," which number about 160. Then comes the species, which is a distinctly different organism whose individuals do not normally interbreed with the individuals of other species.

One of the larger families of birds is the "tyrannidae," or tyrant flycatchers. They are the lowest (oldest) family in the order of passeriformes or perching birds. They are named tyrants because they are generally pugnacious, often chasing and harassing other birds. They feed from an open perch, sallying out to catch insects in flight and returning to their perch.

This is a family found only in the Americas and numbers over 400 different species, most present only in the tropics and subtropics. Over 30 species are found in the United States, but there are only nine flycatchers that breed in New England. One of the distinguishing marks of the flycatcher family is that most have a poorly developed singing apparatus. Many share a simple two note call that is usually harsh and buzzy. But there are exceptions.

Two weeks ago a group of us was walking through the wilds of Plainfield, deep in the Berkshires. Our leader brought us to an old pond surrounded by small alder trees and other wetland bushes and grasses. Here was the domain of one of the tyrants, the alder flycatcher.

The male gave his little song a few times. The traditional transcription is wee-bee-yo, but I agree with the lady who long ago taught me that it sounds just like "three beer." It is a gravelly voice, but gentle enough to distinguish it from the explosive, spluttery song of its twin species, the willow flycatcher.

Soon the bird appeared on the tops of small bare alder branches. It was dark gray on top with a suffusion of olive, wings and tail darker with dull white markings, undersides lighter with a hint of yellow. There was a light eye ring visible. This describes all of the five "empidonax" flycatchers of our area, which are so similar they have a grouping of their own. They are subtly beautiful birds, even if little tyrants. As if to prove it, the bird suddenly leaped into frantic action, uttering a loud, harsh note and chasing a second bird back and forth.

Two weeks later we heard and saw exactly the same call and chasing, but this was in the understory of tall trees. This different flycatcher species was slightly larger and darker, with duller wing marks and no eye ring. It was the eastern wood pewee. We had been hearing his sweet, plaintive song filling the shadows as we walked the trail. The clear, soft, drawn-out whistle of peee-a-weee was pretty good for a family of "songless" birds.

High overhead we saw the bird land on a bare, dead branch, and we realized it was on the edge of its nest. The nest was built cleverly in a cleft that matched the lichen decorating the outer surface of the structure. The camouflage was perfect except for the moving heads that stretched over the edge, showing yellow gapes in hope of food from the parent.

The alder flycatcher has a twin species, the willow flycatcher, and the ranges overlap, with the willow ranging more to the south and the alder residing mostly farther north. These two together are called a "superspecies." They are so alike that they were once considered one species, but the different songs and the discovery that they did not interbreed led to their being split into two.

The eastern wood pewee is also one half of a superspecies. It has an identical twin with a different song called the western wood pewee, and as the names suggest, their range is divided east-west instead of south-north. These two pairs are a snapshot of evolution at work. Each species has evolved to a point of separation, but there is no apparent visual difference between them.

Seeing the differences between organisms is a complex and fascinating challenge for the curious observer of nature.