The Systematic Position of the Cocos Flycatcher

Abstract

-The Cocos Flycatcher (Nesotriccus ridgwayi), traditionally allied with Myiarchus, was recently placed in another subfamily, Fluvicolinae, near Empidonax and Cnemotriccus. A comparative study of the skull and syrinx demonstrates that the nearest relatives of Nesotriccus are Phaeomyias murina and Capsiempisflaveola in the subfamily Elaeniinae, since all three genera share unique derived character states of the nasal septum and of the supporting elements of the syrinx. In 1891 Charles Townsend collected a new flycatcher on Cocos Island, which he later named Nesotriccus ridgwayi (Townsend 1895). He called attention to the zoogeographic and, inferentially, the morphological relationship between his new monotypic genus from Cocos, over 500 kilometers off the Pacific coast of Costa Rica, and an equally interesting flycatcher in the Galapagos Islands, magnirostris, which Ridgway (1893) had recently removed from Myiarchus and placed in its own genus, Eribates. Hellmayr (1927) similarly was impressed with the affinity of Nesotriccus and Eribates and placed both close to Myiarchus, the widespread and successful mainland genus. As recently as 1955, Eisenmann followed Hellmayr in placing Nesotriccus next to Myiarchus in a list of species of Middle American birds. Subsequent recommendations (Swarth 1931; Lanyon 1978; Zimmer, unpubl. notes, Amer. Mus. Nat. Hist.) that magnirostris should be returned to Myiarchus left unresolved the systematic position of Nesotriccus. The alleged affinity between Nesotriccus and Myiarchus was rejected by Swarth (1931) and Zimmer (unpubl. notes) on the basis of external morphology. In a study of cranial anatomy, Warter (1965) concluded that Nesotriccus appears intermediate in structure between the Myiarchinae (s.s.) and Todirostrum, but otherwise is probably more closely related to the former. Ames (1971) reported the syrinx of Nesotriccus to be unlike the syringes of Myiarchus, Empidonax, Contopus, and Cnemotriccus, but made no recommendation as to how Nesotriccus should be classified on the basis of syringeal morphology. In the only definitive classification of the Tyrannidae since Hellmayr (1927), Traylor (1977) removed Nesotriccus from its traditional position among the myiarchine flycatchers and placed it in the subfamily Fluvicolinae, between Empidonax and Cnemotriccus, on the basis of external morphology. At the time that I was completing my revision of the genus Myiarchus (Lanyon 1978), I had decided on the basis of external morphology alone to exclude Nesotriccus from Myiarchus. I doubted that nests of Nesotriccus, when discovered, would be located within tree cavities and lined with fur and feathers, the derived nesting behavior that defines the myiarchine flycatchers (Lanyon 1982, Lanyon and Fitzpatrick, 1983). Subsquently I examined the skull of Nesotriccus and, contrary to the findings of Warter (1965), found it to be very distinct from the skull of Myiarchus. My interest in Nesotriccus was revived with the descriptions of the first known nests (Sherry, in press) and the availability of alcoholic specimens taken by Sherry. I was certain that a careful study of the skull and syrinx would provide meaningful clues to the nearest relatives of Nesotriccus. This paper presents the results of that study.