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Willow Flycatcher (Empidonax traillii) and Alder Flycatcher (Empidonax alnorum) are sibling species that were not distinguished as separate species by the A.O.U. (1973) until the research of Robert Carrington Stein (1958, 1963), showed that two distinguishable song-types, called "Fitzbew" and "Feebeeo," were given respectively by individuals of two distinct biological species. Peterson (1987) recounts how, before Stein's work, he and others began to appreciate the differences as their travels put them in contact with both song-forms. Lowther (1999) describes the difficulties in assigning scientific names to the song-forms, once they were recognized as distinct species. Research subsequent to Stein's has abundantly confirmed and extended his findings. Kroodsma's (1984, 2005) finding that the two species do not learn their songs, but instead develop normal songs regardless of early experience, secured the legitimacy of basing taxonomic decisions on vocalizations. The large body of research on these two species, including the endangered "Southwestern Willow Flycatcher" (E. t. extimus), is synopsized in the BNA accounts for the two species, Lowther (1999) and Sedgwick (2000). They are often referred to collectively as the "Traill's Flycatcher Complex," after one of several pre-split English names.

Despite being very difficult to distinguish visually, the species are readily distinguished by their vocalizations. Most of the main elements in their vocal repertoires are diagnostic. In this document I name and present examples of all these main elements, describe the diagnostic characteristics of each, and explain how they differ from similar sounds. I hope studying the samples and the comparisons presented here will help observers prepare for going into the field, and lead to more accurate and confident on-the-spot identifications. But, at times when, or at places where, the local status of either species is unclear, it would be a good idea to record vocalizing "Traill's Flycatchers," and compare the recorded sounds to the samples presented here. For almost all of the named sounds in the repertoires of these species, geographic and individual variation are insignificant compared to interspecific differences, and these sounds should be sufficiently representative to allow identification to species throughout the range of both species.

Willow Flycatcher is polytypic, with authorities recognizing four or five subspecies (Sedgwick 2000). Southwestern Willow Flycatcher (E. t. extimus) Fitzbew songs (see below for repertoire elements) are significantly different from those of neighboring E. t. adastus (Sedgwick 2000, 2001). Further, eastern Willow Flycatcher (E. t. traillii and E. t. campestris) songs appear to differ consistently, albeit slightly, from the two more northerly subspecies in the West, E. t. brewsteri and E. t. adastus (D. A. McCallum, unpubl. data). My goal for a future version is to document this variation, making it possible, perhaps, to identify a sample to one of three subspecies groups in the species. For now, identification to species is the limit.

Complete repertoires of main elements are presented in two formats. The Reference Page, below, is more exhaustive and descriptive. You can follow links from the Reference Page to the Comparison Page, which presents side-by-side comparison of any two sounds in the repertoires of the two species. In both formats, the same samples are inter-leaved vertically according to acoustic similarity. The arrangement is not meant to imply functional equivalency or evolutionary homology, although comments on both function and homology are included in the text. The Traill's complex has the "roughest" sounds in the entire genus Empidonax. I use "rough" as a general descriptor of sounds that have a "saw-tooth" spectral contour, a rapid and regular oscillation of pitch over a broad frequency range. The first 13 sounds in the reference section all have "rough" parts. The final eight sounds do not. These are "smooth" or "clean" sounding.

Each sound in this document is represented by a half-wave plot, a spectrogram, and text. Half-wave plots display practically all the information displayed by a more conventional oscillogram, with twice the resolution. I use half-wave plots on the assumption that they more readily evoke time-varying loudness for those not accustomed to extracting a sense of loudness from the alternating positive and negative amplitude values of a conventional oscillogram. The wav files have been standardized to a maximum amplitude of 10 volts, which makes it possible to hear the fainter parts of each sound. In most cases the wav files presented for listening have not been high-pass filtered (which filters out the low-frequency noise, such as wind). The clip sounds more "natural" with all frequencies retained. But, I did filter the samples before preparing the spectrograms, and in some cases I have digitally replaced ambient sounds with background noise from the same recording session. I performed these operations to make the spectrograms cleaner. An unavoidable consequence is that the spectrogram does not represent all the background noise you hear on the sound clip.

Each call-type is referred to by an onomatopoeic (or, in one case, acoustically descriptive) name. Names originally intended to describe the aural qualities of a sound, though often failures in that regard, are preferable to functional names because the same sound often has more than one function (i.e., is used in more than one context). I use the name that I judge to have the greatest familiarity, regardless of its success at conveying the aural quality of the sound. In the case of Willow and Alder Flycatchers, Stein (1963) provided useful and accurate spectrographic "descriptions" (equivalent to the naming of taxa) along with onomatopoeic "names" for most call-types. They have been used so widely that it seems best to stick with them. I have revised Stein's name "Pit" to "Pip," in line with widespread practice, and have made a few other minor adjustments, detailed below. Sedgwick (2000) provided further detail on the repertoire of the Willow Flycatcher, with comments on similar sounds of the Alder Flycatcher. He changed two of Stein's names. I comment on these changes. Hereafter, WIFL = Willow Flycatcher, ALFL = Alder Flycatcher.

Text includes recording data, the significance of the call-type for identification of the two species, comments on the spectrographic distinguishing features of the call-type, similar sounds, and common combinations including the focal sound. The generalizations I offer are based on spectrographic and aural study of representative samples from throughout the breeding range of the two species, and include my own recordings, the Borror Laboratory of Bioacoustics online catalogue (http://dmc.ohiolink.edu/cgi/i/image/image- idx?page=index;c=blb), commercially-distributed samplers, and private recordings graciously provided by recordists.

All sounds presented here are copyrighted, and may not be published in any way (including on the WorldWide Web) or distributed for commercial gain, without the express written consent of the copyright holder. The Copyright holder is indicated in the figure legend of the sound. The text and figures of this document are copyrighted (2007, 2008) by Dougald Archibald McCallum, and may not be published in any way (including on the WorldWide Web) or distributed for commercial gain, without the express written consent of Dougald Archibald McCallum.