Black Skimmer is a medium-sized, black and white seabird, essentially resembling a hefty-looking tern. It has a distinctive bill that is laterally compressed, scissor-like, and is asymmetrical vertically, with the maxilla (upper half) being noticeably shorter than the mandible (lower half). This feature distinguishes the species from any other bird in the Americas. The plumage bears a striking contrast: the black upper parts, including the crown, are set against a white underside, including the belly and throat. Overall, the body has a long silhouette with elongated wings, yet coupled with stubby legs. A short tail is covered by the primary feathers when the bird is standing. This squat awkwardness transforms into graceful elegance once the bird takes flight.
From afar, an inexperienced observer might confuse a skimmer with a similarly toned black-and-white shorebird, American Oystercatcher (Haematopus palliatus). However, Black Skimmer is truly unique for its combination of an unmistakable bill plus short bright legs.
The following description is based on Blake (1977) and Gochfeld and Burger (1994); see also Geographic Variation. Sexes similar in all plumages:
Adult (Definitive Alternate): Upperparts sooty black. Forecrown, sides of head, and underparts immaculate white. Upper surface of rectrices black or brownish, outer webs edged with white; under surface of rectrices gray. Wing coverts and remiges black, secondaries tipped with white. Underwing coverts white or pale gray (varying geographically).
Adult (Definitive Basic): Similar to Definitive Alternate, but upperparts with a brown cast, and with a broad whitish nuchal collar.
Immature (First Basic): Crown dark brown, faintly mottled with white. Back and rump sooty brown. Nuchal collar white; this collar can shade into the blackish upperparts, or the transition can be very abrupt and contrasting. Forecrown, sides of face, and underparts white. Rectrices, wing coverts, and remiges as in adult.
Juvenile: Upperparts generally buffy brown, mottled with black; each feather has a black center, and a broad buff border. Forecrown, lores, and suborbital region pale buff; sides of the face and underparts whitish. Greater wing coverts blackish gray or tipped with white. Primaries blackish; primaries 5-8 margined with buff. Secondaries whitish.
A study of skimmers wintering in southern Brazil noted active molt in 62.5% of captured individuals. Molt scores of the primary feathers were correlated with capture date: lower scores were observed in November/December and the highest scores were in March/April (Scherer et al. 2013), indicating that Black Skimmers used available resources to undergo molt during the nonbreeding season.
The presence of skimmers in both breeding and non-breeding plumage during the molting process on the wintering area might reflect a mixture of different post-breeding populations from breeding grounds in middle eastern Brazil, southern Brazil and Argentina (Scherer et al. 2013), as has been observed in South American Tern Sterna hirundinacea (Faria et al. 2010). Scherer et al. (2013) suggested that minimizing molt-gaps (in the wings) may be necessary to maintain the high degree of power and control required for the unique skimming technique that this species uses for fishing.
Examination of museum specimens has shown that both younger and nonbreeding individuals have more gray coloration in the inner webs of the rectrices, as compared to breeding adults (Griscom 1935). This confounds identification, making tails of younger winter specimens of North American niger indistinguishable from those of breeding adults of the southern South American intercedens. (See Geographic Variation).
In adult skimmers, the short tarsi and webbed feet are bright orangey red. The eye-catching bill is two-toned, its bright base matching the leg color, contrasting with black tips. The upper and lower halves of the bill are equal in length at hatching, but by fledging time at roughly 4 weeks post-hatch, the lower portion is almost 1 cm longer than the upper (Gochfeld and Burger 1994).
Scherer et al. (2013) observed that color of the culmen seems to be darker red in males, yet this led to misclassifying one third of individuals compared to molecular sexing. The intensity of red bill coloration is thus not a reliable method for sexing Black Skimmers in the field. This corresponds with an analysis of bill coloration in Roseate Tern Sterna douglallii, which noted that some females had redder bills than males, and found no consistent difference in redness between the sexes (Cormons 1976).
Black Skimmers are sexually dimorphic for several morphological traits. Males are markedly larger than females, although their coloration does not vary (Mariano-Jelicich et al. 2007).
On the most important known wintering area for the species in southern South America — the Mar Chiquita lagoon on the Atlantic Coast of Buenos Aires Province, Argentina — males were 28% heavier than females (mean mass 369.4 g vs 277.6 g), and their culmen length was up to 38% larger (mean 80.5 mm vs 64.8 mm). This study found that both body and bill size were useful traits for predicting sex: body size was a better indicator (97.2% correct classifications) than was culmen length (93.7% correct), although the best overall model combined culmen length + lower bill length to correctly classify 97.9% of individuals (Mariano-Jelicich et al. 2007).
Similarly, during the nonbreeding period on the coast of southern Brazil, males were heavier than females (mean 302.6 g vs 241.4 g). Males were also significantly larger in six other measurements (sexual size dimorphism = 7–26%); head + bill length alone was enough to predict adult sex correctly, for 97.7% of individuals evaluated (Scherer et al. 2013).
In comparison to these southern weights, North American males weigh an average of 365 g vs 265 g for females (Gochfeld and Burger 1994; also see detailed comparisons of other traits from multiple northern and southern locations, within Appendix 3 of their Measurements section).
Based on specimens in the British Museum (Natural History), Griscom (1935) stated that the size of Black Skimmers increased steadily toward the south of their range, such that North and Central American skimmers are smaller than South American individuals. Gochfeld and Burger (1994) also refer to North American niger as being smaller. Yet the masses reported above do not strictly follow that pattern, as skimmers from Brazil have less mass North American skimmers. Of course mass data are influenced by food availability and the point in the annual cycle at the time of study, so this may obscure patterns.
However, the geographical pattern also seems to be unclear in terms of wing length: while wing lengths appear to be longer in Argentinian males (mean 400 mm, Mariano-Jelicich et al. 2007), females there were smaller (mean 339 mm) compared to skimmers in Brazil (mean 388 mm for males, and 363 mm for females; Scherer 2013) and North America (mean 390 mm males, 350 mm females; Gochfeld and Burger 1994). This does not support Griscom's conclusion that all South American skimmers are larger based on his finding that intercedens (the subspecies of southern South America, including southeastern Brazil and Argentina) has "wings of both sexes averaging about 30 mm longer".
Wetmore (1944) measured specimens from the three subspecies and did find that southern birds had larger wings, as shown below, though not 30 mm larger. Perhaps the discrepancies noted here relate to measurement of museum specimens versus live individuals — further studies may help to clarify any latitudinal patterns in size difference:
niger, wing length:
male, mean 380 mm (range 364-401 mm; n = 31)
female, length: mean 342 mm (range 331-362 mm; n = 12)
cinerascens, wing length:
male, mean 392 mm (range 380-416 mm; n = 19)
female, mean 354 mm (range 333-375 mm; n = 10)
intercedens, wing length:
male, mean 395 mm (range 369-425 mm; n = 6)
female, mean 354 mm (range 334-374; n = 8)