Friday, December 21, 2007
Biodiversity Corner: Snow Bunting
When and where seen. Tom Brownrigg saw a group of 16 Snow Buntings in a cornfield along the Acorn Trail in Great Brook Farm State Park on December 5. This is the area of the park on the west side of Lowell Street across from the Ski Center. He noted that, "The birds were skittish when I first arrived, but they settled down and eventually allowed me to approach within about 30 feet."
Snow buntings are not common in Carlisle. Tom has seen them here on only two other occasions, both in November — once at Cranberry Bog and once in the State Park. In the 34 years of the Christmas Bird Count (which is conducted on a single day around Christmas time), there is a single entry made in 1989 of 28 Snow Buntings seen by Steve Spang in the low-lying field between Route 225 and Concord Street less than a mile from the center of town.
Identification. In Carlisle, we are only going to see Snow Buntings in their non-breeding winter plumage. At this time, males and females look very similar. The underside is white and there are large patches of white on the wings. The back and the wings show some black and brown. There is a reddish-brown band across the chest, and feathers on the head and face are tipped with brown. The bill is yellow and the legs are dark gray or black. They are bigger than sparrows and smaller than cardinals but have a relatively large wingspan. A cardinal is almost nine inches from tip of bill to tip of tail and has a wingspan of 12 inches. By comparison, the Snow Bunting is under seven inches long and has a wingspan of around 14 inches.
Safety in numbers. Birds that have unpredictable food sources and therefore must focus a lot of attention on searching for food are vulnerable to predators that depend on the element of surprise. Birds in the Emberizidae family have developed two major strategies to overcome this danger. One, used by Snow Buntings, is to forage in flocks where the group benefits from the many pairs of eyes available to spot predators. This is why Snow Buntings in Carlisle can be seen 16 or 28 at a time. The other strategy is to forage in places where there is ready access to cover. When Tom spotted the Snow Buntings in the corn field, he also saw three Song Sparrows and about ten Tree Sparrows (both members of the Emberizidae family) in the more protected habitat at the weedy edge of the field.
Breeding range. Male Snow Buntings leave their winter range in the northern United States and southern Canada as early as the beginning of April to breed in the high arctic regions of Canada and Alaska where there is still snow cover and the temperatures can be as low as minus 20 degrees F. They find nesting sites in cracks and cavities in rocks and set up territories. The females arrive in May. It is still very cold. The nests have very thick linings of fur and feathers and the female sits on the eggs for almost all the incubation period, during which time the male feeds her. They have mastered the harsh climate, and so far the species is not in danger.
References. Sibley Guide to Birds by David Allen Sibley; Sibley Guide to Bird Life and Behavior by David Allen Sibley; Cornell Lab of Ornithology at www.birds.cornell.edu.
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