Acadian Flycatcher Empidonax virescens. ACFL singing is most combinatorial of all Empids. Dawnsong: Several A notes followed by a chelup, which is merely another A with B tacked on. This sequence (1) repeated continuously. Infrequently-heard Evening Song substitutes a CD phrase for every other AB phrase of Dawnsong. Other combinations, e.g., AD, possible. Post-dawn: AB phrase (chelup, aka hear it SEE it (NDP)) is diagnostic, given at 7-sec intervals. G (identical to YBFL E (2)) is frequently heard, often in series, while C is occasionally performed in series. All empids twitter (F), but ACFL does so frequently. E is uncommon. Yellow-bellied Flycatcher Empidonax flaviventris. YBFL repertoire dissimilar to closest relatives' (Western Flycatcher complex), instead an incongruous amalgam of sounds associated with distantly-related tyrannid species. Dawnsong: One song-type (A, chebunk, tsi-burrt (NDP)) repeated more slowly (1) than sound-alike LEFL A (chebec). Postdawn: chebunk-singing continues at slower rate. Despite distant relationship to LEFL, A is essentially a chebec with "pip" prefix (2), which makes it marginally less mechanical-sounding than chebec. B (Drink, aka chirp (NDP)) is a daytime territorial call; at distance sawtooth part inaudible and remainder sounds like Hairy Woodpecker pik. C (pwee) may be most frequent at dusk; common on winter grounds.; beware nearly identical call of Eastern Wood-Pewee, which is slightly longer in duration. D (peer) and E (pseep) frequent on winter grounds, rare in US/Canada. D is diagnostic, E indistinguishable from ACFL pseep (3). Least Flycatcher Empidonax minimus. LEFL has simplest Empid repertoire. Dawn-song: One song-type (A, chebec) repeated at a rapid but uneven rate (1), continuing after dawn. Chebec is extremely unmusical, and higher pitched (2) than YBFL chebunk, but this may not be discernible. Rapid, insistent cadence is best field mark. D (whit) is only other frequently-used sound. Thinner than WIFL whit, but very similar to whits of 5 other Empids from other regions. B(burrt) and C(pweew) associated with agitated behavior. Alder Flycatcher Empidonax alnorum. Repertoires of ALFL and WIFL similar but easily distinguishable, at least spectrographically. Syntax is the key to field identification of these closely-related look-alikes, which were not split until 1973. ALFL Dawn-song: One song-type: A (feebeeo), repeated slowly (1), continuing after dawn, when the monotony can be broken by insertions of B (Double-peak, pwee (NDP)). C (zweeoo,breer (NDP)) and D (weeoo, pweer (NDP)) are fairly frequently used, sometimes in sequences by themselves. B, C, and D all have significant portions without sawtooth burr (2). WIFL has homologues of B, C, and D, but they are infrequently used. E (pip) is frequently used, often alone, and is close to diagnostic (3), as WIFL uses whit much more often than its pip. In Alaska, confusion with Hammond's Flycatcher pip is possible. See Traill's Flycatcher website for more detail. Willow Flycatcher Empidonax traillii (southeastern). Dawn and daytime singing do not differ: Three song-types: A (fitz-bew, pete's beer (NDP)), B (fizz-bew, free beer (NDP)) and C (creet, greep (NDP)), all burry (1). A and B much longer (2) than all other empid calls save Alder A. Order is quasi-random, although A is more frequent than B or C. Pace (3) unhurried, like an Oscine. Singing with zweeoo (not shown, similar to B but shorter) in daytime or at dusk may be mistaken for Alder because of lack of variety. Whit (D) a little more liquid than other empid whits. Several other call-types not shown are acoustically similar. See Traill's Flycatcher website for more detail.