Metabolus belongs to a group of flycatchers including the genera Pomarea, Mayrornis, Neolalage, Monarcha, and Clytorhynchus. The different plumages of the adults and the immatures are not unusual in this group of genera, this feature being observed in many of the flycatchers of Oceania. Mayr (1933c:1) points out some of the relationships between Metabolus and some of these other genera; he comments that all of them have rather thin bills, in contrast to those of other flycatchers.
Metabolus became established at Truk probably as the result of an independent colonization. It is a well differentiated genus showing little resemblance to Monarcha godeffroyi of Yap. In looking over the genera found in the Pacific area, it appears that Metabolus is closest to Clytorhynchus of the Melanesian region, especially to Clytorhynchus hamlini Mayr, which is resident at Rennell in the Solomon Islands. The bills of these two birds are similar, both being long and thin, with a pronounced hook. In coloration there is some resemblance; C. hamlini has the blackish forehead and chin like the male Metabolus and also the burnt-orange underparts. In C. hamlini, however, the sexes are similar, Metabolus also resembles C. nigrogularis. Like Metabolus, the immatures of this latter species are different in color from the adults.
Monarcha godeffroyi Hartlaub
Yap Monarch
Monarcha godeffroyi Hartlaub, Proc. Zool. Soc. London, 1867 (1868), p. 829, pl. 38. (Type locality, Yap.)
Monarcha godeffroyi Finsch, Journ. Mus. Godeffroy, 8, 1875, p. 50 (Yap); Sharpe, Cat. Birds British Mus., 4, 1879, p. 432 (Yap); Nehrkorn, Journ. f. Ornith., 1879, p. 403 (Yap); Bolau, Mitteil. Naturhist. Mus. Hamburg, 1898, p. 56 (Yap); Dubois, Syn. Avium, 1, 1902, p. 289 (Yap); Reichenow, Die Vögel, 2, 1914, p. 261 (Yap); Mayr, Birds Southwest Pacific, 1945, p. 295 (Yap).
Monarcha godeffroyi Gray, Hand-list Birds, 1, 1869, p. 321 (Yap); Hand-list Japanese Birds, rev., 1932, p. 175 (Yap); Hand-list Japanese Birds, 3d ed., 1942, p. 194 (Yap).
Monarches godeffroyi Hartlaub and Finsch, Proc. Zool. Soc. London, 1872, pp. 89, 97 (Yap); Gräffe, Journ. Mus. Godeffroy, 2, 1873, p. 123 (Yap); Schmeltz and Krause, Ethnogr. Abth. Mus. Godeffroy, 1881, p. 391 (Yap); Kuroda, in Momiyama, Birds Micronesia, 1922, p. 63 (Yap).
Pomarea godeffroyi Wiglesworth, Abhandl. und Ber. Zool. Mus. Dresden, no. 6, 1890-1891 (1891), p. 19 (Yap); Matschie, Journ. f. Ornith., 1901, p. 112 (Yap).