Systematics

Geographic Variation

Populations of the White Ibis sensu stricto are similar throughout the species’ North American range. Potential cline of decreasing size from North America through Central America to South America (van Wieringen and Brouwer 1990), but data needed from intermediate populations. White birds in South America noticeably smaller than those from North America, and South American birds in full breeding condition have the bill more fully black, with breeding males there more likely to have the throat sac fully developed (Hancock et al. 1992).

Subspecies

Hancock et al. (1992:155) summarized extensive morphological data and concluded that the White Ibis was comprised of two subspecies differing in size and in bare part development and coloration when in full breeding condition. “Northern” birds, those that occur from Panama northward (type locality = South California), would be E. a. albus (Linnaeus, 1758), which averages larger and in full breeding condition has the bill only partly black and male gular sacs underdeveloped. Because Hancock et al. (1992) followed Ramo and Busto (1982, 1987) in treating the Scarlet Ibis as conspecific with the White Ibis, the name E. a. ruber (Linnaeus, 1758) was available for birds in South America, which average smaller and in full breeding condition have the bill largely black and male gular sacs well developed. White birds in South America are most commonly seen pairing with scarlet birds (Ramo and Busto 1987), suggesting that the white birds in South America are a color morph of that species. However, with the treatment of the Scarlet Ibis as a distinct species (Am. Ornithol. Union 1998), the South American subspecies of the White Ibis is unnamed (Patten in prep.).

Related Species

The traditional order Ciconiiformes may be an artificial assemblage of long-legged waterbirds (Olson 1978), with ibises (Threskiornithidae) a transitional group with similarities to the cranes (Gruiformes) and shorebirds (Charadriiformes). DNA evidence supports a paraphyletic grouping but nevertheless suggests that storks, ibises, herons, and flamingos are closely related (Sibley and Ahlquist 1990, Sheldon and Slikas 1997). Within the Threskiornithidae, the genus Plegadis is posited to be sister to the genus Eudocimus (Mayr and Short 1970).

Relationship of the White Ibis of North America and Scarlet Ibis of South America remains controversial. Ridgway (1884) considered them one species, but more often they have been treated as two. Hancock, Kushlan, and Kahl (1992) reviewed the controversy and evidence and concluded that the two form a single species. Their decision was based in part on the lack of reproductive isolation between white and scarlet birds in captivity, among feral animals introduced to the range of the other form, and in the wild in South America. Size and color differences do exist between North and South American birds of either color, so the taxonomic allocation of populations of Eudocimus ibises in North, Central, and South America remains a matter for further study.