Here in Stoddard County (central Bootheel of southeast MO) we still have a few crisp adult White-crowned Sparrows in our yard and picking through the vegetable garden.
A quick look at ASM "Species Search" - (http://www.mobirds.org/CACHE/speciesSearch.asp) for White-crowned's in May show that the last reported dates of WCSP (to the CACHE database) in previous years were:
5/17/2009 Columbia Bottom CA
5/17/2008 Columbia Bottom CA
5/19/2007 Columbia Bottom CA
5/20/2006 Fountain Grove CA
5/15/2005 Leach (B K) Mem CA
Disclaimer: These dates are, of course, for conservation area reports only. There may very well be later dates reported to seasonal compilers and/or may be documented in the corresponding edition of "The Bluebird" ASM's quarterly publication.
So the White-crowneds we're seeing now are porbably the last few to migrate through for Spring - given about another week. However, the warbler spring migratory push appears to be about 2 weeks behind, the push of sparrows to be out of the state may also be tardy.
Of course, I digress to those with more experience and knowledge . . . these are simply my humble observations of what is turning into a frustrating spring season for yours truly! Peace. . . !