Bioacoustics of Cacomantis aeruginosus

Diagnosis: main song of two types , one representing the main song in the rest of the complex with elements resembling those of sepulcralis but more variable in form ( Fig. 2 ), and the other a unique and more frequent ‘locomotive’ vocalization of prolonged series of rapidly uttered clipped whistles on the same pitch; basic morph of Wallacean morphotype (see 3.2.2(2) above), but paler with light copper-green dorsal sheen and mid brownish rufous ventral surface, and extensive white toothing on inner vanes of outer rectrices; body medium-sized and tail proportionally long: wing c . 112–122 mm , tail/wing ratio 1.02–1.14 ( Table 2 ); barred females with dorsa dully barred dusky on cinnamon, and ventral white barred clearly black with moderate, if variable, cinnamon wash; juveniles dark-toned with fine markings. The ‘locomotive’ vocalization ( Rheindt & Hutchinson 2007 ; Tebb et al. 2008 ; Thibault et al . 2013 ) has provided grounds for separating this form from sepulcralis at species rank (del Hoyo & Collar 2014; Clements et al. 2019 ; Gill et al. 2021 ). Payne (2005) assessed its populations as intergradient with dorsiventrally greyer Australo-Papuan variolosus , using comments from Hartert (1925) in support. We read those comments as referring instead to putative intergradation between two subspecies of variolosus in the Moluccas (see 4.4.1.1 below). Adding to the original six AMNH specimens of the north Moluccan population ( heinrichi Stresemann, 1931 ), we have now traced a further six in AMNH, RMNH, ANSP, identified by tail/wing ratios greater than 1.02. The resulting series, from Obi, Bacan and Halmahera, is uniform in plumage and long-tailed ( Table 2 ), without any phenotypic sign of intergradation with Moluccan populations of variolosus . If the main song common to the complex is ancestral, then the ‘locomotive’ vocalization in aeruginosus may well have developed as an isolating mechanism against migrant and, in the north, resident populations of variolosus .