Move to a Tidal Marsh, Get a Bigger Beak

January 1, 2005 by Gregory Gough

Along the Atlantic, Gulf, and Pacific coasts of North America, tidal marshes exist in narrow strips or isolated pockets. As sparrows colonize the marshes from inland areas, their bills grow longer and deeper.

This phenomenon holds for the ten species or subspecies of sparrow that inhabit tidal marshes (see table below). Using molecular techniques, scientists have found that some of the sparrows colonized the marshes more than half-a-million years ago while others have colonized fairly recently, less than 10,000 years ago.

Scientists compared each tidal marsh sparrow with its nearest relative and found that although body size was fairly similar, the tidal marsh sparrows had longer and deeper beaks.

The difference in bill size may be due to the food available in tidal marshes. A longer bill is more useful for catching animal prey and less useful for cracking open seeds. Tidal marshes have mud that is exposed by the tides with abundant snails, worms, and crabs. Tidal marshes have few plant species compared to inland areas, and those plants typically do not produce seeds with tough outer shells.

Tidal Marsh SparrowClosest Inland Relative
Belding's savannah sparrowSavannah sparrow
Large-billed savannah sparrowSavannah sparrow
Acadian sharp-tailed sparrowNelson's sharp-tailed sparrow
St. James Bay sharp-tailed sparrowNelson's sharp-tailed sparrow
Saltmarsh sharp-tailed sparrowLeConte's sparrow
Seaside sparrowLeConte's sparrow
Suisun song sparrowModesto song sparrow
San Pablo song sparrowMarin song sparrow
Alameda song sparrowHermann's song sparrow
Coastal plain swamp sparrowSwamp sparrow

This article summarizes the information in this scientific paper:

Grenier, J. L. and Greenberg, Russell S. 2005. A Biographic Pattern in Sparrow Bill Morphology: Parallel Adaption to Tidal Marshes. Evolution, 59(7): 1588-1595

Download scientific paper