Systematics

Ongoing debate over species limits of stilts (genus Himantopus) has led to a confusing array of classifications. This account follows the classification adopted by the Am. Ornithol. Union (1998), who treat all taxa of stilts in the Americas and the Hawaiian archipelago as a single species (H. mexicanus) apart from taxa elsewhere in the world (see Related species, below).

No geographic variation described within North American populations, but no comprehensive study available. Birds of Galapagos archipelago may have shorter tarsi on average (Hellmayr and Conover 1948). Hawaiian birds larger and more extensively black than mainland populations of the Americas (see H. m. knudseni, below).

Three subspecies recognized: H. mexicanus mexicanus (Black-necked Stilt), H. m. knudseni (Hawaiian Stilt), and H. m. melanurus (White-backed Stilt). Each may warrant species rank, although nominate mexicanus and knudseni are thought most closely related and sometimes classified together as a separate species apart from melanurus (Blake 1977). Am. Ornithol. Union (1998) designated each as a subspecies “group” both to acknowledge past treatments of each as a separate species (e.g., Ridgway 1919) and to highlight important differences among them.

H. m. mexicanus (P. L. S. Müller, 1776). Breeds from w. and s. North America south through Middle America and the West Indies to n. South America and Gala-pagos Is. Main concentrations in llanos of Colombia and Venezuela; also in coastal w. Ecuador and Peru (south to Lima; Blake 1977) and scattered sites in An-dean highlands to s. Peru (Lake Junín), where southern limit uncertain (most highland birds resemble nominate mexicanus, but birds with melanurus traits occur seasonally and predominate in the south, e.g. nw. Argentina; Fjeldså and Krabbe 1990). White of forehead does not extend over crown; black of hindneck relatively narrow, not extending to sides of neck, and continuous with back; lores almost entirely white. Measurements (mm) with range and mean (in parentheses) from Ridgway 1919: male (n = 19)—wing-chord 214–232 (222.4), tail 67–76 (72.9), culmen 59.5–70 (65.3), tarsus 98–119 (108.8); female (n = 15)—wing-chord 202.5–218 (211.8), tail 65.5–71.5 (69.3), culmen 61–67 (63.9), tarsus 95.5–109 (103.6). See Appendix 3 for measurements of live birds.

H. m. knudseni Stejneger, 1887 . More or less resident on major islands of Hawaiian archipelago (see Distribution: Outside the Americas, above). Pattern of black and white on crown and neck similar in overall appearance to nominate mexicanus but adults of knudseni show greater extent of black on forehead and cheeks; black of hindneck extends forward to sides of neck and slightly onto upper breast (more extensive on males); posterior half of lores black; tips of uppertail-coverts and rectrices blackish; back and scapulars of female knudseni blacker (blackish-brown instead of brown) than female of nominate mexicanus; white spot above the eye generally smaller, but varies a great deal and commonly absent (Coleman 1981). Averages larger than nominate mexicanus in wing, tail, culmen, tarsus, and mass (see Appendix 3 for measurements of live birds).

H. m. melanurus Vieillot, 1817. Resident in s. South America from e.-central Peru (chiefly east of Andes) and central Bolivia east to se. Brazil (s. Mato Grosso, São Paulo, Rio de Janeiro) and south to s. Argentina (Chubut; Canevari et al. 1991) and s. Chile. In Chile, found from Atacama to Llanquihue, with concentrations from Aconcagua to Concepción (Johnson 1972, Blake 1977). Presumed to intergrade with nominate mexicanus described from ne. Peru and Minas Gerais, Brazil (Hellmayr and Conover 1948, Blake 1977); apparently intermediate specimens cited from ne. Brazil (Short 1975). Captive mexicanus and melanurus have produced a hybrid offspring (Lint 1959). Relationship and pattern of intergradation between mexicanus and melanurus in need of study. Conspicuous white collar across upper back is most reliable character differing from nominate mexicanus; crown usually white (variable, however) with black strap connecting eye to black nape. Size intermediate between nominate mexicanus and knudseni . Measurements (mm) with range and mean (in parentheses) from Blake (1977): male (n = 10)—wing flat 215–244 (233.5), tail 72–86 (79.5), culmen 65–76 (71.1); female (n = 9)—wing flat 215–232 (225.3), tail 68–82 (77.3), culmen 68–74 (70.1).

Worldwide, there are 6–8 taxa of stilts (genus Himantopus), which vary from an all black form to forms with differing patterns of black and white on the head and neck (“pied” forms). Taxonomic arrangement of these is complex and unsettled, partly because their evolutionary relationships are poorly understood, and partly because systematists have used differing notions of species limits. Taxa are essentially allopatric (a few are narrowly sympatric) and separable on plumage and biometrics. Interestingly, a similar taxonomic problem exists among the oystercatchers (genus Haematopus), a more diverse group of black or black and white shorebirds that are closely related to stilts.

Relationship of the Black-necked Stilt (including taxa mexicanus, knudseni, and melanurus) to other stilts in the genus is uncertain. The 3 other taxa of stilts include: himantopus, Black-winged Stilt, Eurasia and Africa (includes meridionalis of South Africa and ceylonensis of Sri Lanka, 2 weakly differentiated forms sometimes recognized as subspecies); leucocephalus, White-headed Stilt (or Pied Stilt), Java east to island of New Guinea and south throughout Australia and parts of New Zealand; novaezelandiae, Black Stilt, endemic to New Zealand. While some combine all stilts into a single species (e.g., Johnsgard 1981), most authorities divide these taxa into 2 species, elevating novaezelandiae to species status and placing all other taxa as subspecies under himantopus (e.g., Pierce 1996). Others accord species status to each taxon except knudseni, which is placed under mexicanus, resulting in 5 separate species (e.g., Sibley and Monroe 1990). All forms may constitute a superspecies (Mayr and Short 1970). These arrangements, however, are more conjecture than fact due to lack of comprehensive study of evolutionary relationships among the various groups. Interestingly, South American melanurus and Australian leucocephalus are more similar in plumage to each other than to geographically adjoining stilt taxa.

The family Recurvirostridae includes the stilts in the genus Himantopus and 2 other genera: Recurvirostra (avocets) and Cladorhynchus (Banded Stilt, C. leucocephalus, monotypic genus restricted to Australia). Phylogenetic analysis of skeletal characters places Himantopus basal (“primitive”) to a clade containing the avocets and Banded Stilt (Chu 1995). This whole group, in turn, is most closely related to the Ibisbill (Ibidorhyncha struthersii) of central Asia, a species that has in the past been classified in the Recurvirostridae. Stilts, avocets, and Ibisbill are all closely related to oystercatchers (Haematopus) within a lineage of plover-like birds placed in suborder Charadrii (Sibley and Ahlquist 1990, Chu 1995, Am. Ornithol. Union 1998). Suggestion that flamingos (Phoenicopteridae) might be derived from stilt-like ancestor (Olson and Feduccia 1980), is refuted by molecular data (Sibley and Ahlquist 1990, Mindell et al. 1997).

Hybridization

An extralimital (possibly escaped) Black-necked Stilt female nested and hatched chicks with a male Black-winged Stilt in the Netherlands (Meininger 1993). Captive Black-necked Stilt and American Avocet have produced hybrids (Principe 1977).