I naively assumed that identifying Eastern and Western Meadowlarks is not impossible. According to my first Peterson guide, although the two meadowlarks are nearly identical, the Western is paler on the back and has the "yellow of the throat edging a trifle farther onto the cheek; best recognized by...song." The Eastern sings "two clear slurred whistles, musical and pulled out: tee-yah, tee-yair (last note 'skewy' and descending." The Western, on the other hand, sings "a variable song of seven to ten notes, flute-like, gurgling, and double-noted; very unlike clear slurred whistles of Eastern Meadowlark." Listen to these calls by clicking on these names: Eastern Meadowlark; Western Meadowlark.
http://www.birds.cornell.edu/crows/mlarkdiff.htmhttp://www.birds.cornell.edu/crows/mlarkdiff.htm) that some rare bird committees do not accept meadowlark identifications without accompanying tapes of the calls.
But the situation gets more complicated. Meadowlarks, where their ranges overlap (as they do in Minnesota), can learn and sing each other's (and other species') calls! (See Davis, Stephen K. and Wesley E. Lanyon. 2008. Western Meadowlark (Sturnella neglecta), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu.bnaproxy.birds.cornell.edu/bna/species/104). Furthermore, these two species occasionally hybridize.
Peter Pyle in his Identification Guide to North American Birds, a book used as an ultimate guide for banders, writes "this is one of the most difficult in-hand species identification problems." According to Pyle, the complete lack of yellow in the malar region identifies female Eastern Meadowlarks, and extensively yellow ones indicate male Weaterns. What is a birder to do? The answer must be to proceed VERY carefully when identifying meadowlarks!