In the last column, I began a discussion of the different concepts of species used by biologists. How do we know what a “good species” is? The column ended with the Biological Species Concept (BSC, for short), in which the ability to interbreed determines if two populations should be combined into one species.

Determining species limits is tricky because hybrids often occur when two presumed species overlap. Orioles provide a nice example of the changing winds of bird taxonomy. Prior to 1983, birders could see Baltimore Orioles in eastern North America and Bullock’s Orioles in the west. Based on the fact that these two species interbreed at their region of overlap in the Great Plains, the AOU Check-list Committee combined the two species into one, called the Northern Oriole. Subsequent work revealed that the two species have very different molting patterns. DNA comparisons indicate that Baltimore Oriole and Bullock’s Oriole are not even their closest relatives. So the Check-list Committee reversed itself and split Northern Oriole back to two species.

Our current taxonomy has some inconsistencies. The American Black Duck and Mallard interbreed freely. Waterfowl biologists are concerned that hybridization with Mallards may be swamping the genetic constitution of the black ducks. Similarly, the Golden-winged Warbler is declining throughout much of its range, due in part to extensive hybridization with the Blue-winged Warbler (a species that is rapidly increasing its range). So despite the ability of these two pairs of species to interbreed freely, we recognize each pair as two separate species.

Compare that situation to the Yellow-rumped Warbler, one of the most familiar warblers nesting in Maine and indeed throughout North America. Two forms of this species are recognized, our eastern Myrtle Warbler and the western Audubon’s Warbler. Male Myrtle Warblers have white throats and male Audubon’s have yellow throats. The two types of yellow-rumps interbreed in a very limited area in the southern Canadian Rockies. Despite the more limited interbreeding than the two species pairs above, the two distinct forms are combined into a single species.

This discussion leads us to consideration of the last species concept, the Phylogenetic Species Concept (PSC, for short). Advocates of the PSC argue that the ability to interbreed is a misguided approach. Rather, these advocates define a species as a group of individuals that all share at least one characteristic found uniquely in that group. If individuals sometimes interbreed with other species but still maintain those unique characteristics, it’s no big deal. Each species has its own history (or phylogeny). Use of the PSC would cause the Myrtle Warbler and Audubon’s Warblers to be split into two species. The limited interbreeding does not alter that fact that the two types of yellow-rumps have unique throat colors.

Unique songs or calls can be used to define a species under this approach. From the work of Jeff Groth and Tom Hahn, we know that there are at least nine populations of Red Crossbills with distinctive flight calls. The different call types do show some morphological differences and differences in choice of cones they feed on. The PSC would result in these nine call types being split into nine species.

Genetic differences can be used to define species using the PSC. If all individuals in a population have a unique sequence of DNA, that information can be used to define a species. Of course, a DNA sequencer is a bit unwieldy for birders to carry in the field!

One upshot of using the PSC is that the number of recognized species of birds will increase. I have seen one estimate that the number of bird species could be doubled to about 20,000. That is one way to work on your life list.

Regardless of the species definition chosen, biologists will continue to be confused for some species or groups of species. Populations change and diverge through time in response to changing climate, increases in competition or changes in predator numbers. To drive this point home, let us end with Herring Gulls.

Herring Gulls occur around the globe in higher latitudes of the northern hemisphere. European Herring Gulls are the lightest in color. They interbreed with the slightly darker American Herring Gulls. American Herring Gulls from Alaska interbreed with even darker Vega Herring Gulls in Siberia. The Vega gulls interbreed with the darker yet Herring Gulls in central Russia and they interbreed with Lesser Black-backed Gulls in Europe. But at the end of this ring, the Lesser Black-backed Gulls and European Herring Gulls can’t interbreed! Where do we draw the species line?

[Originally published on December 13, 2009]