In many cases, closely related species can be distinguished by their songs.
 Select a pair of birds from the list below.  Click on the map of the USA
to hear the songs of the bird that lives there (some regions have neither
of the two species, and you will hear no song from there).  Find the places
where the songs switch between Eastern and Western forms.  Can you think of
a general rule for the biological dividing line between East and West for
North American species?  What feature may represent a barrier for these
species (perhaps a mountain range, river, or prairie)?

All bird songs used in this project, except where otherwise noted, are property of Borror Laboratory of Bioacoustics, The Ohio State University, Columbus, OH 43210