The genus Empidonax includes 15 species of similar-looking small tyrant flycatchers (Family Tyrannidae). Some are easy enough to distinguish visually, but most are not. Out-of-habitat and out-of-range birds therefore require careful documentation to be believed. Fortunately for us humans, Empid vocalizations are sufficiently distinct to make acoustic identification straightforward and sound recordings of them are sufficient documentation. This is especially the case on the breeding grounds.
Five species breed in the moist eastern third of the North American temperate zone, and its extension to the northwest. On these pages I provide synopses of the sounds most useful for distinguishing these five species on the breeding grounds, and perhaps on migration or on the winter grounds in the tropics. These five species represent all four of the major clades of Empidonax, and therefore are about as distant genetically as any sample of five Empids can be, even if they don’t look it. A brief description of the four clades and their relevance to eastern Empids follows. Follow this link to read about the evolution of the entire genus.
Johnson and Cicero (2002) used mitochondrial DNA data to estimate and clarify the phylogenetic tree of Empidonax. They found the genus deeply divided into four branches (clades) that split from each other some 6 mya (million years ago), long before the Pleistocene Epoch and its ice ages. In a companion paper, Cicero and Johnson (2002) showed that Empidonax was closely related to the phoebes (Sayornis), pewees (Contopus) and the tufted flycatchers (Mitrephanes), as had been proposed earlier by Wesley Lanyon. Fjeldså et al. (2018) recently performed an analysis using sequences from both mitochondrial and nuclear DNA. The high resolution of their results, based on a very large sample of species, led them to the conclusion that, “Empidonax represents an assemblage of four well-supported clades, corresponding to those already demonstrated by Johnson and Cicero (2002). There is, however, no strong signal for monophyly of Empidonax, as there is no resolution between these four clades and Sayornis and Contopus/Mitrephanes.” Careful reading of the Cicero and Johnson papers, moreover, reveals that they never explicitly advocated the monophyly of Empidonax. It is possible, therefore, that some empids are more closely related to pewees or phoebes than to some other empids. If this finding is confirmed, it will be necessary to revise the taxonomy of the group. I will discuss this issue at greater length elsewhere. For present purposes -- understanding the differences in eastern Empids -- it is sufficient to stick with current taxonomy.
The four clades of Empidonax, as characterized by Johnson and Cicero (2002) and confirmed by Fjeldså et al. (2018), may someday be treated as separate genera, but I have not researched the nomenclatural history of this group, and am currently loathe to propose new generic names. Instead, I give them monikers below that are linked to their breeding habitat. The five species that breed in eastern North America are highlighted with red text. In order of their divergence from the other lineages, per Johnson and Cicero (2002), the clades are:
It has no close relatives, period. Its nest, which is suspended, like a vireo’s, from a horizontal fork of twigs, is unique in the genus, and indeed in the entire subfamily Fluvicolinae (Cicero and Johnson 2002, Fjeldså et al. 2018). Its breeding habitat, forested places near water, from coastal swamps to mountain torrents, is also unique. It is the only breeding empid in the coastal plain of the southeastern U.S. Its vocal repertoire contains 5 simple sounds that are typical of flycatchers, but it combines them into highly structured singing performances unmatched by any of its relatives except Mitrephanes.
Yellow-bellied, Pacific-slope, Cordilleran, and Yellowish Flycatchers.
These four species nest in shady locations where moss is abundant. They use moss and fibers to build their nest on a shaded ledge, within the root ball of an overturned tree, or under the eaves of a house (like a phoebe). Their ranges overlap minimally, i.e., they are regional representatives of a single adaptive strategy, a statement that is confirmed by the extensive interbreeding of Cordilleran and Pacific-slope Flycatchers where their ranges overlap. My Western Flycatcher website goes into more detail on the intermediate vocalizations of the hybrids. These species and the Yellowish Flycatcher of Central America comprise the “Western Flycatcher Complex.” Their vocalizations are higher and sweeter than those of the other clades, and all have the same repertoire organization and syntax. A recent paper from John Klicka's lab ( Linck et al. 2019). provides more detail on the evolutionary history of this complex. The taxonomic implications are vexing. The eastern representative of the Moss Clade is the Yellow-bellied Flycatcher, a denizen of spruce forest. It diverged from the others 3.3 mya (Johnson and Cicero 2002) to 4.5 mya (Fjeldså et al. 2018:figure 2). The Yellow-bellied Flycatcher’s repertoire bears little resemblance to the rest of the clade; one call is shared with the Acadian, and one with the Least Flycatcher. Not surprisingly, the close relationship of this species to the Western Flycatcher Complex was not appreciated until genetic work by Johnson and Cicero (2002), which has been confirmed by Fjeldså et al (2018).
Alder, Willow, and White-throated Flycatchers.
Another clade with species that are largely allopatric is this assemblage of species that nest in damp, treeless areas with densely packed vertical stems. Their nests are placed in these thickets of willows, alders, or early successional trees. Their voices are as rough as the Moss Clade's are sweet. Their main sounds are also longer in duration than most other empid sounds. Although Alder and Willow are easily distinguished by voice with study, the distinctions between then were not appreciated until the 1950s, leading to the split of the erstwhile Traill’s Flycatcher in 1973. For more on this situation, visit my Traill's Flycatcher website.
Least, Hammond’s, Dusky, Gray, Buff-breasted, Pine, and Black-capped Flycatchers.
The seven species of this group are less tied to moist areas than the members of the other three clades. This may represent an evolutionary innovation that allowed them to diverge ecologically and thereby occupy different habitats in the same geographic ranges. Nonetheless, only one, Least, is found in the east. All four North American species use short rough sounds (as opposed to the long rough sounds of the Stem Clade), but they are readily distinguished using syntactical clues.
Although several species of Empidonax winter in South America (but not the Caribbean), all 15 species nest only in North America. Five nest in the temperate highlands of Middle America, six in the cooler parts of western North America, and four in the cooler parts of eastern North America, including the boreal forest of Canada. Only two, the Cordilleran Flycatcher and the Willow Flycatcher, breed widely in two of those areas. An outlier, biogeographically as well as phylogenetically, is the Acadian Flycatcher of the Water Clade, which nests in the warmer parts of eastern North America. Clades 2, 3, and 4 are represented by at least one breeding species in each of the three geographic areas occupied by the genus. The same is true of the Pewee genus Contopus and the Phoebe genus Sayornis. The four empid clades really do act like separate genera. One other striking geographic pattern is the repertoire organization of the three northernmost species in Empidonax. They -- Least, Yellow-bellied, and Alder, each in a different clade -- are the only species with only one song-type in their repertoires. Now why would that be? Send your hypotheses to web at archmccallum.com.
The sounds and sonograms on the field guide page, together with the text above, are all you need to know to appreciate the wonderful diversity of eastern Empids.
Cicero, C., and N. K. Johnson. 2002. Phylogeny and character evolution in the Empidonax group of tyrant flycatchers (Aves: Tyrannidae): a test of W. E. Lanyon’s hypothesis using mtDNA sequences. Molecular Phylogenetics and Evolution 22: 289–302.
Fjeldså, J., J. I. Ohlson, H. Batalha-Filho, P. G. P. Ericson, and M. Irestedt. 2018. Rapid expansion and diversification into new niche space by fluvicoline flycatchers. Journal of Avian Biology doi: 10.1111/jav.01661.
Johnson, N. K., and C. Cicero. 2002. The role of ecologic diversification in sibling speciation of Empidonax flycatchers (Tyrannidae): multigene evidence from mtDNA. Molecular Ecology 11: 2065–2081.
Linck, E., K. Epperly, P. van Else, G. M.. Spellman, R. W. Bryson, Jr., J. E. McCormacks, R. Canales-Del Castillo, and J. Klicka. 2019. Dense geographic and genomic sampling reveals paraphyly and a cryptic lineage in a classic sibling species complex. Syst. Biol. 0(0):1–11, 2019, DOI:10.1093/sysbio/syz027.