"A pair of jaybirds came up from nowhere"
The jaybird, or blue jay, is a type of corvid
common across the United States. It features heavily in African American folklore and is said to be a servant of the Devil. Folk Beliefs of the Southern Negro
(1926) by Newbell Niles Puckett contains this story:
Every Friday the jaybird.. [visits] hell to take kindling, sand, or a drop of water to the devil. Some say that this grain of sand is a ransom for the souls in hell, who cannot be released until all the sand on the surface of the earth has been carried below; while others take the view that the jaybirds sold themselves to the devil at one time for an ear of corn, and are obliged to take sticks and sand to him every Friday to make his fire hot. (p. 549)
Though most sources state that the bird returns to earth on a Saturday (which April 8th 1928 was), Faulkner, it seems, was familiar with some variant of this myth, for on p. 228 Luster instructs the jaybirds to “Git on back to hell, whar you belong to. Tain’t Monday yit”.