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Body size (see Measurements) increases from the eastern part of the species’ range west to the northern Great Plains and northwestern part of the range, especially among females (46). Males from southwestern populations (New Mexico, Oklahoma, Texas) have the tarsi short but bill long. Wing length, tail length, and width of white terminal band to the tail are greater in the West. Variation is not clinal. Some reproductive traits vary geographically (see Breeding).
No subspecies, in that Tyrannus t. hespericola Oberholser, 1932, which Oberholser (34) recognized as valid, is a junior synonym of T. tyrannus (Linnaeus, 1758) (41). Subsequent research has shown, however, that larger size in the West is a product of clinal variation in size. Additional junior synonyms are: T. carolinensis (Gmelin, 1788); Muscicapa rex Barton, 1799; T. pipiri Vieillot, 1807; T. animosus (Lichtenstein, 1818); T. matutinus Vieillot, 1819; T. intrepidus Vieillot, 1823; T. leucogaster Stephens, 1826; T. vieillotii Swainson, 1831; and T. t. vexator Bangs, 1898.
Along with flycatchers of the genera Myiarchus, Pitangus, Myiozetes, Myiodynastes, Attila—and a host of others—the genus Tyrannus is in the nominate subfamily, Tyranninae (48), of the Tyrannidae, the speciose (> 400 species) family of tyrant flycatchers confined to the New World, particularly the Neotropics. This subfamily is sister to the subfamily Fluvicolinae, which includes the Contopus pewees, Empidonax flycatchers, Sayornis phoebes, Xolmis monjitas, and various other species (49). Tyrannus itself may be near to Empidonomus and Griseotyrannus (50), two tropical genera. On the basis of voice, plumage and behavior, Smith (17) grouped T. tyrannus with three West Indian species, T. dominicensis (the Gray Kingbird), T. caudifasciatus (the Loggerhead Kingbird), and T. cubensis (the Giant Kingbird). These four species lack brightly colored plumage and have vocal repertoires more similar to one another’s than similar to those of other species in the genus.
More recent molecular analyses lacking data only for the Giant Kingbird and Thick-billed Kingbird (T. crassirostris) suggest entirely different relationships (51). The primary clades defining phylogenetic relationships within Tyrannus appear to distinguish primarily tropical breeding from temperate-zone breeding (Eastern Kingbird, Western Kingbird, Cassin’s Kingbird and Scissor-tailed Flycatcher) species groups. Thus, similarity in voice, plumage, and behavior of Eastern Kingbird and West Indian species most likely a result of convergence.