Most taxonomists can be divided into lumpers and splitters. Those who see similarities between organisms as more important than differences tend to be lumpers. Those who see the differences as more important than the similarities tend to be splitters. There have always been lumpers and splitters and the pendulum regarding the definitions of species has swung more than once or twice.
The decisions made by taxonomist are made for taxonomists not birders. Those birders old enough to remember the great lumping of the late 1970's may also remember the anger and frustration expressed over the nationwide shrinking of bird lists that resulted. Juncos, flickers, yellow-rumped warblers were all lumped. In the real world nothing had really changed, Myrtle Warblers were still distinguishable from Audubon's Warblers and Red-shafted Flickers could still be sorted from Yellow-shafted Flickers. But they were no longer "countable" according to the rules of the American Birding Association.
The lumping and splitting of species is a human endeavor. Taxonomy is a reflection of our need to organize the world around us and the grouping things is a mix of the science and philosophy of the times. The prevailing criterion for lumping and splitting in the 20th century was "reproductive isolation", an assessment of the ability to successfully interbreed. By the late 20th century absolute reproductive isolation gave way to "positive assortment", usually reproductively isolated with the occasional dalliance. Technologies that allow us to look at species at the molecular level have strongly influenced the definition of species driving a modern trend toward splitting. We can quantify positive assortment at the genetic level much more easily than we can in the field. Most recent splits have fallen out along the lines of recognizable forms and most birders celebrate these splits. Splits mean additions to one's life-list and that's always a good thing... well almost always a good thing.
In 1980, Ned K. Johnson published a monograph on what were then designated Western Flycatchers called "Character variation and evolution of sibling species in the Empidonax difficilis-flavescens complex". In this paper, Johnson described the differences between interior and coastal forms of the Western Flycatcher. He was able to show that the songs and calls were different and showed statistically significant differences in size, color and habitat choices. He concluded that they were probably distinct species. None of the characters, with the exception of song on the breeding grounds worked reliably for sorting these new species in the field.
In 1988 Johnson co-authored a paper with J. A. Marten that examined the Western Flycatcher complex at the genetic level. This paper found significant genetic differences between interior and coast forms and was able to make a strong case for positive assortment at the largest zone of sympatry in the Siskyou Mts along the Oregon/California border. All of this strengthened the argument for splitting. In 1989, the AOU formally split Western Flycatcher into interior (Cordilleran) and Coastal (Pacific-slope) Flycatchers.
Empidonax flycatchers, in general, are notoriously difficult to sort out. Subtleties of voice, color, bill shape, primary projection and tail wagging all come into play in making an identification, but for most species a reliable field identification can be made. There are recognizable differences that can be mastered by the diligent birder, except it seems for Cordilleran/Pacific-slope Flycatchers which do not appear to be recognizable forms in the field. The closely related members of the Western Flycatcher complex fall into the category of "cryptic" species: species that often defy traditional morphological methodologies for categorization and cannot be easily discerned without specialized audio equipment or a genetics lab. Away from the breeding grounds Pacific-slope and Cordilleran Flycatchers are probably impossible to tell apart in the field.
So, birders that celebrated the addition of species that fell out from genetics work on recognizable forms were frustrated by Pacific-slope and Cordilleran Flycatchers. The differences between songs and calls of each type were beyond the auditory capabilities of many and in those places where the ranges of both species overlap, these two flycatchers would apparently do each other's vocalizations. If the only tool a birder has to sort flycatchers is song and both species might do the same songs, then they are not, at least to birders, recognizable forms and shouldn't be countable as separate species. A surprising number of birders have called for the re-lumping of the Western Flycatcher complex.
The hostility toward this split is actually quite fierce. Some of it comes from so-called "old-school taxonomists" who grew up with the strictures of reproductive isolation and quantitative morphology as the tools of categorization. Most birders are not taxonomists, however. The roots of their hostility must come from somewhere else and I suspect it's a perceived fairness issue. The sport of birding measures success by the size of one's list. People with poor hearing are at an unfair disadvantage if song is the only way to identify a species and the creation of species that no one can identify, well what's the point in that?
The point is, of course, that taxonomy is a tool for biologists. Modern taxonomy models the evolutionary relationships and changes between populations of organisms over time. Taxonomists have an agenda which has nothing to do with what we birders want or need. We birders use the products of taxonomy, but we don't necessarily understand how the products are created or used by biologists.
Many birders have taken microphone in hand and gone out to record Western Flycatcher vocalizations throughout the west. They are collecting data that, when taken together and applied in an unbiased, scientifically credible manner, will help us better understand this complex. But we have to remember, the split was not based solely on vocalizations. Vocalizations are what we birders have seized upon as the mechanism for sorting recognizable forms. Showing that vocalization is undependable as a mechanism for humans to recognize these forms in some or all parts of the range is not the same as showing that they are undependable for the flycatchers. We need to make sure our personal agendas do not get in the way of truly understanding the relationships between interior and coastal forms of this complicated problem taxon.
Johnson, N. K.. 1980. Character variation and evolution of sibling species in the Empidonax difficilis-flavescens complex (Aves: Tyrannidae). Univ. Calif. Publ. Zool. 112: 1–151.
Johnson, N. K.. 1994b. Old-school taxonomy versus modern biosystematics: species-level decision in Stelgidopteryx and Empidonax. Auk 111: 773–780.
Johnson, N. K., J. A. Marten. 1988. Evolutionary genetics of flycatchers. II. Differentiation in the Empidonax difficilis complex. Auk 105: 177–191.
Lowther, P. E. 2000. Pacific-slope Flycatcher (Empidonax difficilis) and Cordilleran Flycatcher (Empidonax occidentalis). In The Birds of North America, No. 556 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Phillips, A. R.. 1994. The known birds of North and Middle America versus the current AOU list. Auk 111: 770–773.
Posted by mbalame at June 17, 2006 06:05 PM