It's all about the food

Several readers have written in recently about sightings of large flocks of our state bird, the

American robin . "Have robins stopped migrating?" they wonder.

Not exactly. Despite being heralded as the "harbingers of spring," some robins have always remained in the northern United States through the winter. As a species, they are what we might call a "partial migrant." Some fly farther south than others in winter; across the continent, the picture is more of an overall shift in the species' range rather than of a complete vacating of the premises, so to speak (as we see, for example, in most of the colorful

warblers, orioles and tanagers ). Thus, the sighting of a flock of robins is not unusual in winter.

The major determinant of whether and how much birds migrate is food. Although we had quite a bit of snow for December, there are still plenty of fruit and seeds outside, and most of it is readily available. On Shepard Road in Norfolk, there are at least a dozen ornamental

crabapple trees laden with ripe red fruit, which is why a flock of pine grosbeaks has been hanging out there for several weeks now.

(This reporter, along with many other birders, was fortunate to see about 30 of these beautiful, robin-size

finches last weekend. The males are suffused with rose-red, the females gray with golden-buff highlights. Interested readers are encouraged to go check out the birds for themselves, observing all the rules of property and politeness, of course.)

In the case of the robin, we might add one caveat: Numbers of wintering birds have generally increased over the last 40 years, though with occasional fluctuations. The record year on the Lakeville-Sharon Christmas Bird Count was 2001, when more than 30,000 robins were counted - an order of magnitude higher than in any other year! One assumes this is a reflection of our milder winters over the same period, with the resulting abundance and availability of food.

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dark-eyed junco at his feeder. Like many songbirds, juncos show a considerable amount of variability in plumage, and albinism is unusual, but not rare.

Naturalist and writer Fred Baumgarten may be reached at fredb58@sbcglobal.net. He blogs at thatbirdblog.blogspot.com.