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.