The first season Cowbirds in Arizona and South Texas differ both morphologically and vocally.
Males respond to song playbacks.
Cowbirds in the secondary contact zone are hard to find and hard to catch. Sample sizes are too low to conduct a true transect. Dialects and Species Recognition Receiver systems in different species can be so far diverged that inferences about the process of divergence cannot be drawn. But Dialects provide an opportunity to study the process of divergence in receiver systems within a species. Dialect boundaries are narrow.