- Order: Charadriiformes
- Family: Chionididae
The Snowy Sheathbill is sturdily built with stout legs and feet, and dense, all white plumage over a thick gray underdown. Their feet lack webbing and they do not have aquatic adaptations, so they avoid deep water and forage on land (Forster 1996). The conical, stout bill has a greenish sheath, and the face has pink caruncles with thick, fleshy eyerings. Adults are differentiated from younger birds based on bill sheath size and the extent of facial caruncles. Adults have blunt, black carpal spurs, which are used in territorial interactions. Sexes are similar in appearance. The male averages about 15% heavier than the female and has a larger bill and sheath, but the mass and body dimensions of males and females may overlap. The Snowy Sheathbill closely resembles the Black-faced Sheathbill (Chionis minor), but the Black-faced Sheathbill has all black facial features, and the ranges of the two species do not overlap (Forster 1996). See Similar Species.
The Snowy Sheathbill is similar to the Black-faced Sheathbill (Chionis minor), which has distinctive black facial caruncles, bill and sheath (Shirihai 2008). The face of the Snowy Sheathbill is more colorful, and its sheath is less conspicuous than the Black-faced Sheathbill (Forster 1996). The two species also do not overlap geographically, as Black-faced Sheathbill is confined to the Indian Ocean. The Snowy Sheathbill perhaps also could be confused at a distance with a gull (Larus) or Kelp Goose (Chloephaga hybrida), but the sheathbill has a smaller size, pigeon-like gait and swift movements (Shirihai 2008).
The Snowy Sheathbill is a rather silent bird (Shirihai 2008), and its call is not well known (Forster 1996), although at times it may utter "short throaty, crow-like harsh calls" (Shirihai 2008).
During courtship, the Snowy Sheathbill also makes a "harsh and throaty crowlike call and low muttered sounds accompanied by mutual deep bowing" (Watson 1975).
Detailed Description (appearance)
Adult: Plumage entirely dull white with a gray underdown (Murphy 1936, Jones 1963). There is no seasonal variation in plumage or soft parts (Forster 1996). The female is smaller than the male but is similar in external appearance (Forster 1996), although the male "may show more papillation on the bare skin of the face" (Murphy 1936).
Juvenile: Similar to adult, but has a weaker bill, less wattles around its bare facial areas and a smaller sheath. Juveniles have pointed tips to the primaries, which are rounded in the adult; this character usually is useful only for the inner and unworn primaries due to wear (Watson 1975).
Hatchlings: Downy young is brown. It is slightly lighter on the head and tufted, with white on the chin and below the eyes. While developing the white juvenile feathers, the feather down is subsequently pushed out on the tips of the white feathers. At about 12 days, the chick begins to grow a dark bluish gray body down in the bare areas between the feather tracts. This gradually creates an overall gray appearance just before the white feathers begin to show (Watson 1975).
Iris: brown (Murphy 1936, Blake 1977).
Bare skin below the eye is a flesh color (Murphy 1936, Blake 1977), and the papillae are "mostly white" (Murphy 1936) or yellowish (Blake 1977).
Bill: Black or brown distally, and the base and bony sheath of the bill are greenish and yellow, "sometimes with reddish or violaceous hues" (Murphy 1936).
Tarsi: Gray or bluish (Murphy 1936, Blake 1977).
Males average larger than females, but the "high overlap in mensural data does not permit sexual identification based on any lone variable" (Shirihai 2008); the sexes can be distinguished using discriminant function analysis, for which the most important variables are measures of bill shape and tarsus length (Favero 2001).
The following linear measurements are from Blake (1977):
Total length: 380-405 mm
Wing length (flat), males: mean 253.5 mm (range 246-260 mm, n=6)
Wing length (flat), females: mean 240.5 mm (range 232-255 mm, n=12)
Tail length, males: mean 122.5 mm (range 112-135 mm, n=6)
Tail length, females: mean 121.5 mm (range 104-133 mm, n=12)
Exposed culmen, males: mean 32.5 mm (range 30-34 mm, n=6)
Exposed culmen, females: mean 30.8 mm (range 30-32 mm, n=12)
Additional linear measurements from Murphy (1936):
Wing length, males: mean 255 mm (range 246-260 mm, n=4)
Wing length, females: mean 242.2 mm (range 234-255 mm, n=10)
Tail length, males: mean 126.2 mm (range 112-135 mm, n=4)
Tail length, females: mean 124.2 mm (range 118-133 mm, n=10)
Exposed culmen, males: mean 32.5 mm (range 30-34 mm, n=4)
Exposed culmen, females: mean 30.8 mm (range 30-32 mm, n=10)
Tarsus, males: mean 45 mm (range 42-47 mm, n=4)
Tarsus, female: mean 41.5 mm (range 39-44 mm, n=10)
Wingspread, males: 810-840 mm
Wingspread, females: 760-805 mm
Favero (2001) presented mensural data on live sheathbills (unlike the specimens measured by Murphy 1936 and Blake 1977), including the following:
Wing length (chord), males: mean 252.3 mm (range 229-268 mm ± 7.0, n=52)
Wing length (chord), females: mean 243.3 mm (range 218-258 mm ± 7.6, n=46)
Exposed culmen, males: mean 33.6 mm (range 31.2-36.4 mm ± 1.2, n=52)
Exposed culmen, females: mean 31.7 mm (range 27.5-38.7 mm ± 1.8, n=46)
Tarsus length, males: mean 45.2 mm (range 33.3-52.7 mm ± 3.1, n=52)
Tarsus length, females: mean 42.3 mm (range 37.8-47.8 mm ± 2.6, n=46)
Mass: males, mean 719 g (range 640-810 g, ± 40 g, n=40); females, mean 635 g (range 560-720 g, ±43 g, n=37). Data from Favero (2001).
"A complete molt begins in adults in January before the chicks have fledged and lasts into April or May in the South Shetland Islands and into June on South Georgia" (Watson 1975).
Sheathbills are the only bird family with a breeding range that falls entirely within the Antarctic and Subantarctic, and it is the only common and widespread land-based bird in that region. The family has two species that are have a similar appearance and habits, but that do not overlap geographically. The Snowy Sheathbill breeds on the Antarctic Peninsula and subantarctic islands of the Scotia Arc, and many birds migrate to Patagonia, Tierra del Fuego and the Falkland Islands in winter. The Black-faced Sheathbill (Chionis minor) lives on four widely separated island groups year round close to the Antarctic Convergence in the southern Indian Ocean. These four populations each usually are classified as separate subspecies. Since the last ice age ended 10,000 years ago and the pack ice "bridge" linking the four island groups retreated southwards, the subspecies of Black-faced Sheathbill are probably genetically isolated (Forster 1996).
Fang, Emerson D.. 2010. Snowy Sheathbill (Chionis albus), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: http://neotropical.birds.cornell.edu/portal/species/overview?p_p_spp=144116