Birds photographed in Iowa. Western on 12/21/03 and Eastern on 6/7/03. Note the low-contrast head pattern on the Western compared to the more boldly contrasting head pattern on the eastern. Western has a yellow sub-moustachial stripe compared to white on eastern. Eastern more streaked on the flanks. They can occasionally learn each others songs but I believe the calls remain distinctive. Western is pale gray-brown overall compared to rufous overall for the Eastern. The Eastern also has distinctly more white in the tail.