The American Ornithological Society, or AOS (formerly the AOU), has just published the 59th supplement to its Checklist of North and Middle American Birds. This supplement, which used to be occasional, has become annual because new information is accruing so fast and producing regular changes in our understanding of bird relationships.
As far as Missouri goes, the 2018 supplement has produced no species splits or lumps that would affect our list. In fact, the only one that affects North America at all is the split of the White-collared Seedeater into two species, with the one found in Texas now being named Morelet's Seedeater.
Some of our names, however, have changed. This includes one English name: "Gray Jay" has reverted to "Canada Jay," which was that bird's name in days of yore (and Peterson's first three editions).
The other name changes are at the genus level, due to a breakup of one genus into two or three to reflect new information about relationships:
1) The woodpeckers of the genus Picoides have been divided into two genera: the American Three-toed and Black-backed Woodpeckers remain in Picoides, while the Downy, Red-cockaded, and Hairy (in that order) are placed in a new genus, Dryobates, along with some others. Their species names remain the same; e.g., the Hairy is now Dryobates villosus instead of Picoides villosus.
2) Similarly, the genus of "grass-running" sparrows, Ammodramus, has been split up to reflect the fact that these birds are not all derived from the same lineage. The only one left in Ammodramus is the Grasshopper Sparrow, while Baird's and Henslow's Sparrows are now in a new genus, Centronyx, and LeConte's and Nelson's are in another new one, Ammospiza, along with the coastal Seaside and Saltmarsh Sparrows. Again, all of them keep their specific names.
Another type of rearrangement involves the order in which birds are listed within a family, without affecting them at the species or genus level. This reflects how closely related they are to other birds in the same family.
3) Some of our raptors have been reorganized in this way: no changes in species or genera, but some changes to the sequence. Most curious is the fact that White-tailed, Swallow-tailed, and Mississippi Kites are now considered to belong to three different subfamilies—i.e., they aren't closely related! (White-tailed and Swallow-tailed still happen to end up next to each other in our list, but Mississippi is far from them.)
4) A similar rearrangement of subfamilies among the flycatchers has substantially reordered our list without creating any changes that would raise an eyebrow.
For those who want more detail, including the changes for Middle America, here is Michael Retter's explanation on the ABA web site: