This post is a follow-up to the previous one in which I discussed some of the recent decisions by the North American Checklist Committee. This committee of ornithologists makes decisions on whether some species should be split and others should be combined into a single species.

Many were surprised that the Committee did not vote to split the Yellow-rumped Warbler into three species. The votes on this issue was not unanimous. Today, we will discuss three different definitions of species, each of which has its champions.

The first efforts to classify life on earth hinged on a morphological definition of species. Individuals that look alike are combined into a single species. This definition works pretty well for most birds. However, pitfalls lurk.

Consider the bird once called the Traill’s Flycatcher, a common breeding bird in Maine. Careful observations revealed that some male Traill’s Flycatcher give a three-noted song, “three-bee-o” and others gave a sneezy, two-noted “fitz-bew”.  Females respond to only one of the two song types. We now recognize two species, Alder Flycatcher and Willow Flycatcher. But morphologically, the two species are nearly identical. Such similar species are called sibling species. A morphological definition of a species fails us here.

The other side of the coin involves species that are highly variable. The point is made by a wonderful portrait of the 7-foot, 2-inch basketball player Wilt Chamberlain and the 4-foot, 11-inch jockey, Willie Shoemaker ( With the differences in height and skin color, one could easily imagine alien visitors regarding the two men as belonging to different species rather than rightly placing them in a single, highly variable species.

One of the great biologists of the 20th century, Ernst Mayr, devised an alternative definition of species, his biological species concept. A biological species is a group of interbreeding individuals that do not breed with any other species.

The ability to produce offspring is Mayr’s standard for defining a species. The Willow Flycatcher and Alder Flycatcher example above pose no problem as the song types of the two species prevent interbreeding.

Revisiting the Yellow-rumped Warbler question, the Committee decided to not recognize the western Audubon’s Warbler and the eastern Myrtle Warbler because they interbreed freely in a very narrow zone of overlap.

But the biological concept species has some shortcomings. First, what do you do when a species shows a patchy distribution? In western North America, a species once called the Scrub Jay, occupies the role of our Blue Jays in the east. But Scrub Jays also occur in southern Florida. Are the Florida and western Scrub Jays the same species? They never come into contact so we do not know if they can interbreed.

Once considered a separate species, the Committee voted years ago to recognize them as two species, the Western Scrub-Jay and Florida Scrub-Jay, based on differences in social behavior.

The other shortcoming is that interbreeding occurs frequently in many species. Ducks are perhaps the best example. American Black Ducks and Mallards freely interbreed. Hybrids show no loss of vigor or fertility. Mallards interbreed with many other species too ( But there is no outcry for merging Mallards with other hybridizing ducks.

The most recent definition of a species is called the phylogenetic species. Proponents of this concept seek to identify groups that all stem from a common ancestor.  A phylogenetic species is defined on a feature uniquely found in all individuals. A distinctive song, behavior or structure might suffice. Sometimes, the unique feature is a particular section of DNA.  In such a case, field identification is problematic.

Reasonable people can disagree and our understanding of bird relationships and species limits is always subject to revision. Science by nature is provisional. Rather than being frustrated by decisions of the Committee, I take pleasure in the complexity of nature.

[First published on July 30, 2017]