Two subspecies of swamp sparrow nest in the mid-Atlantic states: the southern swamp sparrow and the coastal plain swamp sparrow. Scientists compared the songs of southern swamp sparrows from the mountains of Maryland with the songs of coastal plain swamp sparrows from the tidal marshes of Delaware.
The songs differed between the two populations in syllable composition, repertoire size, trill rate, and frequency bandwidth.
Scientists played recorded songs of the different subspecies for territorial males and the birds reacted more strongly to songs of their own subspecies.
These two subspecies have likely been separated since the last Ice Age, about 10 to 15 thousand years ago. Their appearance does differ, with the coastal birds being darker, but their DNA is very similar. A divergence in song suggests that these two subspecies are on their way to becoming separate species.
This article summarizes the information in this scientific paper:
Liu, Irene A., Lohr, Bernard, Olsen, Brian and Greenberg, Russell S. 2008. Macrogeographic Vocal Variation in Subspecies of Swamp Sparrow. The Condor, 110(1): 102-109.
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