Move to a Tidal Marsh, Get a Bigger Beak
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 Sparrow | Closest Inland Relative |
|---|---|
| Belding's savannah sparrow | Savannah sparrow |
| Large-billed savannah sparrow | Savannah sparrow |
| Acadian sharp-tailed sparrow | Nelson's sharp-tailed sparrow |
| St. James Bay sharp-tailed sparrow | Nelson's sharp-tailed sparrow |
| Saltmarsh sharp-tailed sparrow | LeConte's sparrow |
| Seaside sparrow | LeConte's sparrow |
| Suisun song sparrow | Modesto song sparrow |
| San Pablo song sparrow | Marin song sparrow |
| Alameda song sparrow | Hermann's song sparrow |
| Coastal plain swamp sparrow | Swamp 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
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