Morphological and vocal variation among subspecies of the black-faced sheathbill

Abstract

The Black-faced Sheathbill (Chionis minor) is a sedentary and polytypic species. Four allopatric subspecies are known, each breeding on one archipelago in the Southern Indian Ocean. To evaluate the degree of isolation of these four subspecies, morphometrics and vocalizations of adult birds of Iles Kerguelen and Crozet were compared with those of the other localities (Prince Edward and Heard Islands). Two groups were distinguished (Prince Edward-Crozet and Kerguelen-Heard) on geographic and morphological criteria. In the eastern group (Kerguelen-Heard), corresponding to higher latitudes, sheathbills were larger and heavier, following Bergmann's Rule. The sheathbills from Iles Kerguelen also had a lower-pitched voice than those from Iles Crozet, consistent with their larger body size. Moreover, the birds from the southernmost locality (Heard Island) had a shorter culmen, consistent with Allen's Rule, but longer tarsi and deeper sheaths. Within the western group (Prince Edward-Crozet), and at Iles Kerguelen, there also was variability on a microgeographical scale. Differences between subspecies of Black-faced Sheathbill therefore could be due not only to environmental correlates of latitude, but also to possible genetic drift. The four subspecies are allopatric and do not differ in their breeding schedule or in their general behavior and diet, suggesting that differentiation may be recent and mainly due to geographical isolation.