Vigors listed three known species: "Elanus melanopterus", "E. furcatus", and "E. Riocourii". But he noted that the latter two had more forked tails and probably didn't have nails that were rounded underneath. The following year he gained access to specimens of the fork-tailed kites and split them from "Elanus" into a separate genus, "Nauclerus". In 1931, Peters used the subfamily Elaninae, listing its members as "Elanus", "Chelictinia", and "Machaeramphus". He placed "Elanoïdes" in subfamily Perninae, and "Gampsonyx" with the forest falcons in Polyhieracinae. In the 1950s, several authors found "Gampsonyx" was related to "Elanus" rather than the falcons, based on morphological features and molt schedule. Lerner and Mindell describe the Elaninae as: "Kites noted for having a bony shelf above the eye, "Elanus" is cosmopolitan, "Gampsonyx" is restricted to the New World and "Chelictinia" is found in Africa". This is in contrast to the Perninae, which are: "Kites mainly found in the tropics and specializing on insects and bee or wasp larvae, all lack the bony eye shield found in the Elaninae". Comparisons of sequences for certain mitochondrial marker genes indicate that some elanine kites split early from the rest of the Accipitridae. Wink and Sauer-Gurth found that "Elanus" was less related than the osprey and secretary bird (which are often placed in a separate family), but noted that this was not strongly indicated. However, Lerner and Mindell found that the osprey was less related, but "Elanus leucurus" was basal to the other Accipitridae. Negro and colleagues have discussed convergent traits between kites in the genus "Elanus" and owls, such as a lower acidity of the stomach and some specialized flight feathers otherwise not found in diurnal raptors. Lerner and Mindell also found that "Elanoides forficatus" grouped with Perninae, such as the type species "Pernis apivorus" and the Australian endemics "Lophoictinia" and "Hamirostra".