In the last column, I wrote about the scientific classification of birds.  I pointed out that the families and orders into which birds are classified are pigeonholes defined by scientists.  The only taxonomic unit that is defined by nature is the species.  Recognizing species is not as easy as one might think.  We’ll begin to explore this prickly subject today.

A recent scientific review showed that there are over 60 definitions of species.  I will collapse this list to three basic concepts and discuss the strengths and weaknesses of each.

The morphological species concept is based on the idea that all members of a species should be similar.  Of course similar is a subjective adjective.  I like to show my introductory biology class a portrait of the basketball player Wilt Chamberlain next to the jockey Willie Shoemaker.  An alien would scarcely suspect that these two mean, differing so much in height and skin color, belong to the same species.  Strike one for the morphological species concept.

Sibling species, species that are deceptively similar, also pose a problem for the morphological species concept.  Prior to 1973, certain flycatchers with eye-rings and wing-bars were identified as Traill’s Flycatchers.  Fieldwork showed that some male Traill’s Flycatchers gave a sneezy fitz-bew song and others gave a fee-bee-o song.  Researchers found that some Traill’s females responded only to the fitz-bew songs and others only to the fee-bee-o songs.  Traill’s Flycatcher is actually two very similar species: the Willow Flycatcher and the Alder Flycatcher.  Distinguishing the two species in the field is nearly impossible and even in-hand identification is not always possible.  Strike two for the morphological species concept.

Ernst Mayr, one of the greatest biologists of the 20th century, developed the biological species concept, defining a species as a population of interbreeding organisms that are reproductively isolated from other populations.  This concept is widely held and is the predominant view of the members of the American Ornithologists Union Check-list Committee that rules on taxonomic changes for the birds of the Americas.

The critical aspect of this concept is the ability to breed with other individuals to produce viable offspring.  Different species should be unable to hybridize; they should be reproductively isolated even if they are found in the same area.  Isolating mechanisms include songs, displays and infertility that prevent members of different species from mating or producing viable young.

Two problems arise with his concept.  First, how do you treat populations that are geographically separate from each other?  Consider the scrub jay complex.  Scrub Jays are very common birds west of the Rocky Mountains, essentially occupying the ecological role of our Blue Jays.  But there is a population of Scrub Jays in south Florida.  Also, the Scrub Jays on Catalina Island, off the coast of Southern California, look a bit different from western Scrub Jays.  How can you apply a test of the ability to interbreed for populations that are not in contact with each other?  The answer is that it becomes a judgment call.  Formerly, these jays were all treated as one species, the Scrub Jay.  More recent work, based in part on DNA differences, has split the scrub jays into the Florida Scrub Jay, Western Scrub Jay and Island Scrub Jay.  We still don’t know if these different populations can interbreed.  Strike one against the biological species concept.

Another problem is that a lot of birds do hybridize.  More than 10% of bird species care capable of creating viable offspring with members of other species.  Ducks are the most notorious birds in this regard with over 400 hybrid combinations known.  Mallards hybridize with over 50 species of ducks and geese.  Wood Ducks hybridize with over 40 species.  Over 10% of the American Black Ducks and Mallards in New England are hybrids.

Hybrids occur regularly in wood warblers as well.  Lawrence’s Warbler is a well-known hybrid between a Golden-winged Warbler and a Blue-winged Warbler.  The Cincinnati Warbler described by Audubon is now known to be a cross between a Blue-winged Warbler and a more distantly related species, the Kentucky Warbler.

Even though hybrids often are capable of reproduction, in some cases their mixed-parentage offspring are not fertile.   Some hybrids capable of reproduction may have more difficulty in securing a mate than pure-breed individuals.

But many hybrids can secure mates and produce offspring that are fertile.  In such cases, ornithologists measure the proportions of hybrid to “pure” offspring where two populations overlap.  The two populations are judged to be the same species if hybrids are common.  In the next column, we will see this rule in action.  I will also present a third, more controversial, species concept.

[Originally published on November 29, 2009]