Black-billed Streamertail

Frontiers of Taxonomy: Why Are There Two Types of Streamertails?

Red-billed and Black-billed Streamertails share Jamaica, with Red-billed exclusively occupying most of the island except the easternmost 5% or so, where Black-billed predominates.  They overlap to a limited extent, as small numbers of Red-billed occur within Black-billed’s tiny range.  They are obviously very similar to one another, but also readily distinguishable by sight—at least in the bill color and general coloration of the males.  Their vocalizations and mating displays also differ.  More subtly, their bills differ in shape, as Red-billed’s is somewhat broader than Black-billed’s.  Their overall measurements are close, but Red-billed averages slightly larger than Black-billed.

In light of the clear differences between them and their persistence as distinct forms, it is peculiar that they are widely regarded as conspecific: the Streamertail (Trochilus polytmus).  As of August 2019, this remains the official classification of the pertinent taxonomic authority, the American Ornithological Society.  It seems certain that the AOS will eventually recognize the two as separate species—this does not appear to be a close case by contemporary standards.  The AOS’s anachronistic position is a vestige of “Lumping Fever,” which overwhelmed taxonomists in the 1900s but has gone out of fashion and is unlikely to return as long as biologists continue to value biological diversity.

Disagreement over the species status of Red-billed and Black-billed Streamertails raises a central issue in the delineation of species: the significance of interbreeding.  The streamertails present an analytically clean example, almost as if they were a laboratory experiment designed to decompose this issue.  They are extremely similar (similar enough to plausibly be one species), but also easy to tell apart (different enough to plausibly be two species).  They are confined to the same island, where they occupy the same general habitats, but mostly in different locations, although no physical barrier separates them.

On the surface, the streamertails appear to be sibling species, western and eastern counterparts, but this conclusion is complicated by the fact that they have been reported to interbreed at least occasionally.  Based on these reports, ornithologists governed by the “biological species concept” decided that the streamertails must be races of a single species.  The “biological species concept” reduces the definition of a “species” to a set of organisms capable of producing fertile offspring.  This is among the classic criteria for determining what constitutes a species, but it is inadequate to describe the dynamic diversity of the living world.  Taxonomists who reflexively apply the concept seek evidence of any interbreeding and prioritize it above all other factors—though in cases such as the streamertails it becomes clear that this approach fails to comprehend the core issue of taxonomy: speciation.

Rigid application of the “biological species concept” to the streamertails shows how an exception can sometimes swallow the rule.  Through reliance on a single factor, advocates of this approach ignore the main question: why are there two types of streamertails on Jamaica?  Having coexisted in the same limited real estate for tens or hundreds of thousands of years while capable of interbreeding, why do they remain distinct?  If they are the same species, then why have they not blended into a single gene pool?  The answer must be either that they recognize one another as different so interbreeding is rare (i.e., assortative mating) or that their hybrids are disadvantaged and fail to propagate over many generations (i.e., natural selection), or both.  Either response is tantamount to a description of speciation.

Contrary to the “biological species” analysis, occasional instances of interbreeding streamertails actually confirm their separateness by showing that the birds themselves experiment with their species limits.  In other words, exceptional cases of hybridization prove that no merely circumstantial barrier prevents the two populations from merging, yet these populations have remained distinct through the millenia.  So evidently the distinction between them is intrinsic and the inescapable conclusion is that they are separate species: Red-billed and Black-billed Streamertails.