Friday, November 14, 2008
If you are walking around the edges of open fields in Carlisle in the spring and fall, you are likely to see a Savannah Sparrow, a fairly common migrant. It can be confused with the similar-looking and much more common Song Sparrow, which is found in similar habitats. When in doubt, look carefully at the bird and then consult a field guide.
Another way to distinguish the two species is to learn their songs. The Latin name Passerculus means “small sparrow (passer)” and sandwichensis refers to Sandwich Bay on Unalaska Island in the Alaskan Aleutians, where a different subspecies is found. The common name “Savannah” was given by early ornithologist Alexander Wilson, and does not refer to its preferred habitat (appropriate, except for a difference in spelling), but to Savannah, Georgia, where the original type specimen was collected.
Uncommon breeding bird here
Savannah Sparrow is not a common breeding bird in Massachusetts, probably because the grassland habitat it requires has been disappearing to development and reforestation. Carlisle birder Alan Ankers has evidence, based on hearing the male’s song on May 31 and June 22 that Savannah Sparrows probably breed at Foss Farm; they might also breed at Towle Field, but the evidence for Towle is less strong. Ankers has conducted grassland bird surveys at Hanscom Field, where there is a large Savannah Sparrow breeding population.
According to Ken Harte’s Christmas Bird Count data for the Carlisle sector of the Concord count circle, Savannah Sparrow has not been seen since the Carlisle counts began in 1973. However, they have usually been seen in small numbers in other towns within the count circle. Harte suspects that if the Carlisle sector had included Foss Farm, we might have seen them during the Carlisle counts.
In fall and winter Savannah Sparrows eat mainly plant food, especially the seeds of grasses and also weeds such as ragweed and chickweed. During the spring and summer breeding season their diet shifts to include a high percentage of animal food, including insects and spiders. We have not seen Savannah Sparrows eating commercial birdseed mixtures that are popular with other sparrows such as Chipping, Song, White-throated Sparrow, and Dark-eyed Junco.
The female Savannah Sparrow constructs her nest made of grasses on the ground, usually in a slight depression, well concealed by a canopy of grasses or other vegetation. In Massachusetts, the eggs (usually four to five) are laid between late May and late June. The eggs hatch in about 12 days, and the young fledge eight to 12 days after hatching. Studies in Vermont found that haying before June 11 resulted a 99% rate of nest failure, and that haying after June 21 resulted in a 2.7-fold increase in fledglings per female per year (reference 1).
The writer thanks Alan Ankers and Ken Harte for helpful comments. A special thanks to Alan for permission to use his excellent photo, taken in Medford, Mass. (where the sparrows look a lot like ours).
1. Wheelwright, N.T. and J.D. Rising. 2008. Savannah Sparrow (Passerculus sandwichensis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/045
2. Baird, J., in Massachusetts Breeding Bird Atlas, Petersen, W.R. and Meservy, W. R. (editors), Univ. of Massachusetts Press, 2003.
3. Veit, R.R. and Petersen, W.R. Birds of Massachusetts, Massachusetts Audubon Society, 1993.
4. Martin, A.C., Zim, H.S., and Nelson, A. L. American Wildlife & Plants: A Guide to Wildlife Food Habits, Dover Publications, 1951.
5. Gruson, E. S., Words for Birds, Quadrangle Books, 1972. ∆
© 2008 The Carlisle Mosquito