Species limits usually lean on morphology or genetics. Meadowlarks lean on acoustics. The eastern meadowlark's song is a small set of clear, slurred whistles; the western meadowlark's is a longer, gurgling, flute-like phrase. In the overlap zone the plumage differences are subtle, involving the extent of yellow on the malar region and the pattern of the outer tail feathers, and are unreliable in the field.
The birds treat the songs as decisive. Where the two occur together they generally hold separate territories and pair assortatively, and hybridisation is rare relative to the extent of range contact. Song functions as a premating barrier, and this is a textbook case of behaviour maintaining a species boundary that appearance would blur.
The more urgent fact concerns numbers. Grassland birds have shown the steepest declines of any North American bird group, driven by conversion of grassland to row crops, earlier and more frequent hay cutting that destroys ground nests, and loss of fallow field margins. Eastern meadowlark is assessed as Near Threatened.