What happens when 3 types of swamp sparrows meet up in the Spring and Fall in Delaware? Do they mix? Do they mingle?
Surprisingly, the answer is no. Dr. Russell Greenberg and his team from the Smithsonian Migratory Bird Center (SMBC) combed marshes, roadsides, and underbrush at the Woodland Beach Wildlife Management Area in Kent County, Delaware, from mid-September to May over a 2 year period, looking for swamp sparrows.
These medium-sized birds can be found in marshes across North America and Canada hunting for seeds and insects in the mud and plants near the water's edge.
The scientists from the SMBC were specifically searching for the coastal plain swamp sparrow (Melospiza georgiana nigrescens) in Delaware, but they also found the southern swamp sparrow (Melospiza georgiana georgiana) and the northern swamp sparrow (Melospiza georgiana ericrypta) in the area during their survey. They discovered fairly quickly that the 3 types, or subspecies, of sparrows generally don't mix.
Why? It might be because the coastal plain subspecies is so specialized.
The coastal plain subspecies of swamp sparrow only lives in a narrow range of habitat along the coast of the United States between New Jersey and the Carolinas. They can be found in these tidal marshes on the coast year-round, both during breeding season and in non-breeding periods.
On the other hand, northern and southern swamp sparrows have a much broader migratory range. These two subspecies may only be seen in the coastal area of Delaware as they migrate or while they are in search of certain types of food. But because the coastal plain swamp sparrow has adapted so well to life in the tidal marsh, the size, shape, and structure of the bird, or its morphology, may have changed.
Does this mean the coastal plain swamp sparrow may be evolving into its own species? That remains to be seen.
The study also found the different subspecies of birds didn't stay in the area for the same amount of time. The coastal plain swamp sparrow left earlier than the other 2 species. During the month of October, all the coastal plain swamp sparrows left. And like clockwork, during the month of April, they all returned.
With the other 2 subspecies, migration was spread out over a longer period of time. So how did the coastal plain swamp sparrows time their arrival and departures so well as a group? The answer can be found by looking at climate records. The smaller subspecies of coastal plain swamp sparrows were reacting as a group to the weather—in particular, freezing temperatures.
This article summarizes the information in this scientific paper:
Patterns of Seasonal Abundance and Social Segregation in Inland and Coastal Plain Swamp Sparrows in a Delaware Tidal Marsh. 2010. Russell Greenberg,Brian J. Olsen, and Matthew A. Etterson. The Condor 112(1):159-167.
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