Friday, October 22, 2004
When and where seen: The northern harrier was spotted by Jean Keskulla on October 7 or 8 at the Cranberry Bog. We are likely to see harriers in Carlisle only during migration from September to November, and then again in April.
Distinguishing characteristics: This grassland hawk is most easily recognized by its behavior. It flies low over fields and marshes, usually within just a few feet of the ground, alternately gliding and flapping. When gliding low, the wings are held in a V-shape. It will systematically process a hunting area by flying back and forth in a pattern called 'quartering'. This low-flying hunting behavior is nicely captured in a series of photos at the online Fermilab Raptor Gallery at www.pbase.com/gaocus/fermi_raptors. The males are gray above and pale below with dark spots, while the females are dark brown above and pale below with brown streaks. The one Jean saw was a juvenile which is an orange color below. Males, females and juveniles all have a white rump. The white patch on the back, just above the tail, can be seen when the bird is flying away from you. The northern harrier is a slender bird with a long tail and a wing-span of 43 inches. It is about the same length from tip of beak to tip of tail as the red-tailed hawk (18 to 19 inches on average) but it weighs less than half as much. It has yellow eyes and an owl-like face.
The fields are alive with the sounds of voles: The northern harrier has an extremely well-developed ability to detect sound, particularly for a diurnal species. An experiment by William Rice of Oregon State University showed that the harrier's ability to locate prey by sound exceeded that of other hawks and was matched only by owls which can find prey in complete darkness. The arrangement of feathers on the face, which give the harrier its owl-like appearance, are part of the sound-gathering system. A major prey item of the harrier is the vole, a creature not noted for being highly visible or loud. Keenly honed senses of sight and sound come in handy.
Breeding: Harriers' breeding grounds of open fields and pastures are being lost to development and to reforestation. As of 1989, there was an estimated 30 to 50 breeding pairs in Massachusetts, all of them in the islands off Cape Cod where there is extensive open moorland. Today the harrier is classified as threatened in Massachusetts - and is also on the endangered species lists in other states.
References: David Allen Sibley, The Sibley Guide to Birds; Richard Veit & Wayne Petersen, The Birds of Massachusetts; National Geographic Field Guide to Birds of North America; and the Hawk Conservancy Trust of the U.K. online at www.hawk-conservancy.org.
The only requirements for the Biodiversity Corner are that the subject exists in the wild and was seen in Carlisle. Anyone can write the column, or send a note about your sighting or a photo to Kay Fairweather at 392 School Street, Carlisle MA 01741 or to firstname.lastname@example.org.
More than leaves are changing color
Occasionally animals show colors other than their traditional and familiar colors. On September 20, Ruthann and Jan Tore Hall reported a black squirrel at their bird feeder. Despite its color, it is actually a gray squirrel. The black variant is quite common in more northern latitudes. With our current large population of squirrels, the likelihood of a black one occurring is greater. On October 12, I saw a skunk at the lower playing field at the Banta-Davis Land with tawny yellowish stripes. The tip of its tail was starkly white and the short little stripe on its snout was white but the other usually white parts were definitely of autumnal tones.
© 2004 The Carlisle Mosquito