William S. Clark
In 1959, American ornithologist Kenneth C. Parkes noted that the plumage of the black-shouldered kite is similar to that of the black-winged and white-tailed kites, and proposed that all three were subspecies of a single cosmopolitan species "E. caeruleus"—much like the peregrine falcon ("Falco peregrinus"). Researchers William S. Clark and Richard C. Banks disputed this, pointing out the differences in anatomical proportions such as wing shape and tail length, and hunting behavior ("E. caeruleus" rarely hunts by hovering, unlike the other two species) and proposed the species be separated again in 1992. They are regarded as distinct in the IOC World Bird List.