We had 2 beautiful male Mourning Warblers near the Chubb Trail in West St. Louis CO. this morning so start to watch for them.  According to Birds of MO, this is a little early for Mourning but someone I know thought they had one last Saturday too. 
Pat Lueders, Webster Groves, MO
The Audubon Society of Missouri's Wild Bird Discussion Forum
List archives: https://po.missouri.edu/archives/mobirds-l.html