Some folks have wondered aloud whether these early birds (who are surely not going to get the worm) are responding to global warming. This is not the explanation, simply because the explanation is a different one. So far, it appears that global warming is affecting animals in the parts of the globe where local climates and related conditions are changing most rapidly, notably the polar regions. Trends in animal habits and behavior have multiple causes, and these are very hard to determine.
More experienced bird watchers have surmised that perhaps more and more kinds of birds are extending their ranges northward and staying here for the winter. This northward trend in year-round residency is well-known to ornithologists and avid birders, and goes back to the early twentieth century at least. The litany of birds that have moved north over the last century is long, and includes charismatic species such as eastern cardinal, mockingbird and Baltimore oriole, and more recent arrivals such as red-bellied woodpecker and Carolina wren, which are now well-established year-rounders.
The American robin and the eastern bluebird are a bit different. Both are migrants, but both are also year-round residents in the Hudson Valley, and have been for a long time. But both disappear from our dooryards, farms, suburbs and cities during the coldest times of year. This absence is the result of local migrations to more supportive winter habitats that may be only a few miles from their summer haunts.
Consider first the robin. This bird's winter regime is somewhat complicated. For one thing, there are two different types of robins in our area during the winter, whereas in summer only one of them is present. The American robin, with the unfortunate scientific name of Turdus migratorius, has been set upon by sagacious systematists who have divided the species into seven subspecies. Five are western, and two occur in our region. The familiar robin of spring, T. migratorius migratorius, is the state-wide, year-round resident. The less familiar (probably mostly unknown) T. migratorius nigrideus breeds in Canada but counts New York State as part of its overwintering territory; it flies north for the summer.
So where are all these winter robins? Probably most of them are down along the Hudson River, in dense forest thickets and deep swamps, or in large stands of evergreens or tracts of shrubland. Robins flock in winter, so they are concentrated in these secluded areas where they can take shelter from storms and find berries and overwintering insects in abundance. With branches and needles to catch much of the snowfall, they can find patches of bare ground where they can catch that early worm, especially near groundwater springs that thaw the soil.
The eastern bluebird (Sialia sialis) is less familiar than the robin to most people, but more of a prize sighting. Often considered rare, the bluebird was in serious decline in the Northeast until the last quarter of the 20th century. Decimation by DDT, competition from invasive house finches and starlings, and habitat loss as farms fell to development or reverted to forest, bluebird numbers fell to critical lows in the 60s and 70s. Conservation efforts that include preservation of rural landscapes and the proliferation of special bluebird nest boxes across the countryside have brought this lovely bird back from the brink.
Bluebirds seek thickets for winter shelter, as robins do. But accounts of bluebirds killed by late winter storms suggest that this bird is not so hardy as its larger cousin, the robin. This may be true, but the bluebird's habits are a bit different from those of the robin. Bluebird migrations are apparently not very organized, flocks are looser, and individuals will move farther south in colder winters. They nest earlier in the spring than robins, and their habit of returning to preferred breeding areas in late winter, when they may be caught by harsh storms, is unfortunate. But good nesting sites are hard to come by, so it seems that with bluebirds, when the time comes to fly north in earnest, there's no going back. ++