The flycatchers of the genus Myiarchus are a difficult taxonomic group within a family noted for its difficult genera, such as Colztopus, Elaenia, and Empidonax. There are some 14 to 18 species of Myiarchus in the whole of temperate and tropical America, including the West Indies and the Galapagos archipelago. A remarkable uniformity of coloration, a lack of appreciable sexual dimorphism, and interspecific overlap in most mensural characters have led to considerable discrepancy in their systematic treatment. The diagnostic characters that have been used in the past to indicate relationships have often been inadequate and unreliable. The author has undertaken a revision of the genus, based in large measure on field studies of systematically critical breeding populations. This initial paper reports the findings relating to one geographically discrete group, the Middle American populations of the crested flycatchers of the species Mykchus tyrannulus, with special emphasis on the status of the so-called Ometepe Flycatcher (Myiwchus brachyurus) of Central America. Consideration is given to the vocalizations which may serve to supplement morphological characters in discerning taxonomic relationships. HISTORICAL BACKGROUND The crested flycatchers of Mexico have had an eventful nomenclatural history which need not concern us here. Suffice it to say that the populations of eastern Mexico are now known as coop& (Deignan, 1949) and that magister applies to those of western Mexico (see fig. 7 for ranges of forms considered here). The two forms were formerly considered as races of a distinct Mexican species, Myiarchus mexicanus (Nelson, 1904 ; Ridgway, 1907), until Hellmayr ( 1927), the Fourth Edition of the A.O.U. Check-list (1931) and Griscom (1932) “lumped” them with the geographically distinct and polytypic Myiarchus tyrannulus of South America and the Lesser Antilles. These Mexican populations have since been regarded by most workers as conspecific with the disjunct South American group, although not without question (Eisenmann, 1955:67). This author follows Hellmayr, but reserves critical judgment on the matter pending completion of field studies on the South American populations. A new species of Myiarchus was described by Ridgway (1887) from Central America, within the area of the hiatus between the ranges of me&anus and tyrandus. He called this form brachyurus and, on the basis of its larger size and shorter tail, he considered it specifically distinct from Myiarchus nuttingi which he had described from the same region five years earlier. Allen (1892) contested this specific distinction, commenting that the supposed differences were more likely due to individual variation. This may have influenced Ridgway, who later (1907) reduced brachyurus to racial status under nuttingi. Nelson, in his monograph ( 1904), retained the species status of brachyurus. The specific distinction between brachyurus and nuttingi was clearly and adequately summarized by Bangs (1909) and this treatment has been followed by all subsequent workers. Dickey and van Rossem (1938) were impressed with the apparent intermediacy of their El Salvador specimens between brachyurus of Nicaragua and Costa Rica and tyrannulus from Mexico, stating that “there would now seem to be little doubt that it is simply a geographical form of tyrannulus.” This treatment was accepted by Wetmore (1944) and later followed in the Fifth Edition of the A.O.U. Check-list (1957). In his, manuscript for a forthcoming volume of Peters’ Check-list, Zimmer likewise considered