Conformed Flycatchers

A quote I have often had cause to refer to—I believe it originally came from Toby White of Palaeos.com—is that "organisms are under no obligation to speciate with regard to the convenience of taxonomists". For birdwatchers in North America, perhaps no group more embodies this principle than the flycatchers of the genus Empidonax. These small members of the hyperdiverse New World family Tyrannidae comprise fifteen recognised species that have become notorious for the difficulty in telling them apart.

Immature alder flycatcher Empidonax alnorum, copyright Cephas.

The species of Empidonax are uniformly olive brown above, lighter below, with pale rings around the eyes and bands on the wings. They are inhabitants of woodlands (more on that in a moment) and watch for flying insects from a perch, making short flights to capture prey. Though individual species are generally similar in their feeding habits, they are often specifically distinct in their preferred habitats. A molecular (mtDNA) analysis of Empidonax species by Johnson & Cicero (2002) identified four likely clades within the genus with members of a clade each differing in their specific breeding range. Species found in the US and Canada often migrate long distances and closely related species may be found close together outside their breeding ranges (references to ranges below refer to breeding ranges). Species found in Mexico and Central America are more likely to migrate only short distances or be resident year-round.

Acadian flycatcher Empidonax virescens, copyright Aitor.

The Acadian flycatcher E. virescens seems to be relatively isolated from other members of the genus. This species is found in shady forests near water in the eastern US and Canada. Its nest is a cup made from plant fibres suspended in a horizontal branch fork, and it lays lightly speckled eggs.

The yellow-bellied flycatcher E. flaviventris, yellowish flycatcher E. flavescens, Cordilleran flycatcher E. occidentalis and Pacific slope flycatcher E. difficilis form a clade of species that tend to have more yellowish underparts than other members of the genus. Their nests are mossy cups constructed on a protected ledge or crevice. Members of this clade tend to be found in relatively damp forest areas, such as boggy areas of boreal forests in the case of E. flaviventris, or shady canyons in the case of E. occidentalis or E. difficilis. A notable exception is the Channel Islands population of E. difficilis which is found in more open woodlands than its mainland counterparts. Empidonax occidentalis and E. difficilis are found in the western United States with E. difficilis occupying coastal regions and E. occidentalis found further inland. Until fairly recently, the two were confused as a single species; they are almost indistinguishable morphologically but can be separated by their calls.

Least flycatcher Empidonax minimus, copyright Mdf.

The white-throated flycatcher E. albigularis, alder flycatcher E. alnorum and willow flycatcher E. traillii form a clade of species nesting in damp thickets. Again, it was only fairly recently that the more northerly E. alnorum was distinguished from the more southerly E. traillii.

Finally, the remaining species form a clade whose members lay eggs without speckled markings. They are often relatively dark compared to other Empidonax; the black-capped flycatcher E. atriceps of Costa Rica and Panama stands out for the sooty-black coloration of the head. They often inhabit relatively open forest, often at higher altitudes.

Johnson & Cicero (2002) suggested that the largely allopatric (non-overlapping) breeding ranges of species within clades of Empidonax reflected speciation as a result of isolation in glacial refuges during the ice ages. As the ice retreated, the now-distinct species expanded their ranges but excluded each other where they met. Differences in mating calls between related species dissuaded interbreeding. Physical appearance, meanwhile, remained frustratingly monotonous.

REFERENCE

Johnson, N. K., & C. Cicero. 2002. The role of ecologic diversification in sibling speciation of Empidonax flycatchers (Tyrannidae): multigene evidence from mtDNA. Molecular Ecology 11: 2065–2081.