Among serious birders the name for the Mississippi kite is Ictinia mississipiensis Wilson (ick-TIN-ih-ah mis-ihsip-ih-EN-sis). The generic designation is from the Greek word for kite, which aptly describes its smooth, graceful soaring flight. It's also commonly called a "blue kite" and a "mosquito hawk."
In his journals Custis mentioned having seen about three dozen different birds, but this is the only one he described.
A species of Falco which I have not seen described. --Cere [the wax-like membrane on the upper beak through which the nostrils open], lores [the space between the eye and the bill] and bill black; legs yellow; head and neck blueish white; body and wing coverts [small feathers covering the bases of the longer feathers in the wings and tail] lead colour; quill and tail feathers black-brown,--each tail feather with white stripes extending half way across; claws black; belly blueish; wings below with white & ferruginous [rust-colored] spots; inside fulvous [dull brownish yellow]. 14 inches long.
Inexplicably, he neglected to give it a binomial name, so Alexander Wilson, his competitor for the job of naturalist with the Red River expedition, took credit for discovering and naming it while visiting William Dunbar's plantation at Natchez in 1810, and published his information in his American Ornithology the following year.
The Lewis and Clark expedition was north of the Mississippi kite's once extensive range, and so did not see any.
Based on Flores,J&SE, 234-235 and note 99