American White Ibis - Taxonomy

Taxonomy

The American White Ibis was one of the many bird species originally described by Carl Linnaeus in the 1758 10th edition of his Systema Naturae, where it was given the binomial name of Scolopax albus. The species name is the Latin adjective albus "white". Alternative common names that have been used include Spanish Curlew and White Curlew. English naturalist Mark Catesby mistook immature birds for a separate species, which he called the Brown Curlew. Local creole names in Louisiana include bec croche and petit flaman.

Johann Georg Wagler gave the species its current binomial name in 1832 when he erected the new genus Eudocimus, whose only other species is the Scarlet Ibis (E. ruber). There has long been debate on whether the two should be considered subspecies or closely related species, and the American Ornithologists' Union considers the two to be a superspecies as they are parapatric. The lack of observed hybrids was a large factor in the view that the species were separate.

However, in a field study published in 1987, researchers Cristina Ramo and Benjamin Busto found evidence of interbreeding in a population where the ranges of the Scarlet and White Ibises overlap along the coast and in the Llanos region of Colombia and Venezuela. They observed individuals of the two species mating and pairing, as well as hybrid ibises with pale orange plumage, or white plumage with occasional orange feathers, and have proposed that these birds be classified as a single species, which has been followed by least one field guide. Hybrid ibises have also been recorded in Florida, where the Scarlet Ibis has been introduced into wild populations of American White Ibis. Birds of intermediate to red plumage have persisted for generations.

Ornithologists James Hancock and Jim Kushlan also consider the two to be a single species, with the differences in plumage, size, skin coloration and degree of bill darkening during breeding season forming the diagnostic characters. They have proposed the populations recontacted in northwestern South America after a period of separation, and that the color difference is likely due to the presence of an enzyme that allows uptake of pigment in the diet. They have raised the question whether white-plumaged birds of South America are in fact part of the ruber rather than the albus taxon, and acknowledge that more investigation is needed to determine this.