We looked out the back window this morning and saw a flock of about 15 black-capped chickadees passing from treetop to treetop in a southerly direction. A glance at our feeders showed that the local neighborhood individuals still seemed in residence, and that they didn’t pay the slightest attention to the group hop-scotching through the yard.

Many people grow up thinking that birds fall into only two distinct categories when it comes to migration: either they do or they don’t. But it’s not that clear cut. Yes, there are some bird species in which the entire population migrates south. Birds like bay-breasted warblers, wood thrushes, eastern wood-pewees, and many others vacate their North American summer ranges for Central or South American winter ranges.

The other side of the migration spectrum — the species that we generally would call non-migratory — can be a bit harder to describe with absolutes. In our area, we would say that spruce grouse and ruffed grouse are non-migratory, but in some parts of their range in mountainous terrain, even these species may show relatively short-distance migratory movements from higher elevation summer ranges to lower elevation winter ranges.

Which brings us back to Maine’s state bird, the black-capped chickadee, which any school kid will tell you is a species that stays around all winter. Based on general observation, you would likely be compelled to say that the black-capped chickadee is a non-migratory species. However, although not well understood, it’s clear that some individuals migrate south and others do not, and that the proportion of those that migrate tends to be greater in the more the northern parts of the breeding range. The flock of black-capped chickadees that we saw moving quickly south through our neighborhood was likely a group migrating south from the southern boreal regions of Canada, perhaps Quebec.

There are a number of species that people would think of as non-migratory that show a similar pattern. Blue jays are a great example. It is very common to look up into a clear blue autumn sky anywhere in the northeastern U.S. and see dozens, sometimes hundreds of blue jays streaming southward on migration. American crows likewise leave the northernmost parts of their range in Canada and migrate south in the thousands to join those noisy winter flocks that we see in some parts of Maine.

There is one other migration category that doesn’t fit into the simple migrant or non-migrant dichotomy. These are the species that migrate very long distances only under certain conditions. Large numbers of birds like the dark-eyed junco and the white-throated sparrow may stay in coastal Maine well into December or January if conditions remain mild and the ground stays snow free. But as soon as a big snow storm or extended frigid temperatures set in, the bulk of them will quickly move farther south.

Others, like the well-known “irruptives” including finches like common redpolls and pine siskins, will move south in large numbers in some years but not others and generally follow a two-year cycle. Snowy owls may descend in numbers more intermittently, and some, like the great gray owl, may only show an appreciable presence this far south once every 20 to 30 years—and when they do, there’s a good chance you’ll hear about it!

Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology. Dr. Wells is one of the nation's leading bird experts and conservation biologists. His grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, formerly of the Cornell Lab of Ornithology, is a senior director at the Natural Resources Council of Maine. Both are widely published natural history writers and are the authors of the book, “Maine’s Favorite Birds.”