I think Nelson's Sparrow is one of those species that we, as a community, are just learning to look for and identify.
I'm reminded of Ross' Goose and Thayer's Gull. Soon after I moved to Missouri in 1972, Ross' Goose was #2 on the Burroughs Audubon Society's "10 Best List" for a particular year. Now, we expect to see one whenever we take a few minutes to look carefully through a large flock of Snow Geese - i.e. one Kansas City area birder may see the species 8-10 or more times in one year! (And I have seen one in summer). Population expansion with with the great explosion of Snow Goose numbers may account for some of them, but they were here all along!
I forget the year when the Slaty-backed Gull showed up near the bridge to Alton, Ill., but quite a few of us traveled to see it and became acquainted with Thayer's Gulls on the same trip. Previously, there were no records for Thayer's Gull in Kansas. About the same time, Jim Cable saw and photographed Kansas' first Thayer's Gull. Within a year, there were 7 more records in Kansas. Now, although not nearly so common in Kansas as Ross' Goose, Thayer's Gull is reported several times annually in that state and has showed up at Smithville Lake in western Missouri quite a few times. (I have seen Thayer's Gull at Smithville Lake 4 times since 1992).
I predict that as our Missouri birding community becomes more familiar with Nelson's Sparrow, not only will there be a lot more reports -- the extreme dates will also be extended. 
Bob Fisher
Independence, MO
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