We’ve had our first real snow this season. Those of us who feed birds have stocked feeders, put out suet, and waited for the birds to arrive.

However, in this some of us were disappointed. Comments posted on birder e-mail lists, as well as those made personally to me, all contain the same question: Where are all the birds?

Some people report the number of birds visiting their yards is drastically reduced compared to other years; others say there are no birds at all, and their feeders remain full day after day.

There could be several reasons for this. The first is that chickadees, nuthatches, and blue jays – usually assumed to be residents year-round – are actually more migratory than was thought. Birds in our area may move farther south, while birds from up north may move here in response to severe weather and food availability. If the latter is the case, it may take awhile for the newcomers to find feeding stations. If the resident birds are still around, they may be selecting a different part of their range to forage in.

In addition to this, consider that this cold season has been pretty mild so far. This year’s Christmas bird count has proven this. Many migrants, which should have left long ago, seem to “have lingered past prudence,” as Maine Audubon’s Judy Markowsky said. Ample wild food crops of nuts, seeds, and berries may still be available, and birds will be sure to take advantage of this, as well as seek the shelter of the trees, shrubs, and grasses that produce the food.

This brings me to the next factor: the lack of good shelter and roosting places in many people’s yards. These are absolutely essential in winter. A thick screen of evergreens, a wild hedgerow, even a pile of dead boughs and branches in a protected corner of your property are all things that will encourage birds to visit your yard more frequently.

Roosting boxes also are appreciated and used by many birds, but perhaps the biggest attractant is a source of unfrozen water. This last requirement will attract even birds you would normally not see to your yard.

Lastly, many people have asked if the West Nile virus could be the cause of the reduction in bird populations. There is as yet no clear answer to this. Afflicted crows and blue jays have received the most media attention, but according to the Centers for Disease Control and Prevention, more than 110 species of birds have been infected.

How can we keep track of wild birds to determine if the virus is a threat to them? One way you can help is to become involved with the Cornell Lab of Ornithology’s “Project Feeder Watch.” This is a landmark citizen-science effort that has been in place for several years. Through it, scientists have been able to track bird populations across the United States and Canada. For more information, visit the lab project’s Web site at www.birds.cornell.edu/pfw/, or call the lab at 800-843-2473.

Chris Corio, a volunteer at Fields Pond Nature Center in Holden, can be reached at fieldspond@juno.com