An American robin provides an ideal example of how not all migrations are the same the American robin. Some leave Maine and may only go as far as Massachusetts. Carlos Osorio/Associated Press

Humans have a propensity to organize, to put things into boxes. People who try to pigeon-hole nature often do not fare well with their efforts. I still remember the admonition of one of my biology professors who said that nature has not stake in being classified.

As this fall migration proceeds, we can see how our attempts to sort out different migration strategies may lead us astray. One dichotomy that is often used for migratory birds is to contrast long-distance with short-distance migrants. Usually, a North American long-distance migrant describes a species that moves south out of the continent to Central America, South America or the Caribbean islands. A host of our nesting birds fall into this category: ruby-throated hummingbirds migrating to Central America, red knots to Argentina and scarlet tanagers broadly across South America.

In contrast, short-distance migrants typically move south to areas with more clement winter weather but do not leave the continent. Some of our breeding birds, like the American robin, travel only short distances. Some may only go as far as Massachusetts. Other short-distance migrants like our yellow-rumped warblers may go as far as the coastal regions of the mid-Atlantic states. American tree sparrows nest on the taiga and high altitudes and migrate to Maine for our “mild” winters.

We can see the messiness of this system by comparing a short-distance migrant like Lincoln’s sparrow, which may winter in the Rio Grande Valley (that’s where I saw the species for the first time) with a long-distance migrant like the rose-breasted grosbeak that spends its winter in Mexico. Hardly a difference there in distance traveled.

Let’s compare that same grosbeak to another long-distance migrant, the arctic tern, some of which migrate from pole to pole every spring and fall. The differences in migration length are huge, but both are classified as long-distance migrants.

Another contribution to the inadequacy of our classification is the phenomenon of partial migration. Our field sparrows nesting in southern Maine head south to the Carolinas or beyond for the winter. The field sparrows that nest in North Carolina do not migrate so there is a mix of resident and migrant sparrows in the winter. It turns out that the resident sparrows outcompete the migrants and have a higher survivorship over the winter.

Blue jays are a local example showing partial migration. In some years, some but not all our blue jays migrate south in the fall. In years where oaks have high acorn production, the jays stick around to enjoy their favored food. In years where acorn production is meager, many jays move south.

American goldfinches, purple finches, red-breasted nuthatches and black-capped chickadees may show partial migration as well.

As surely as temperatures decrease as you travel toward higher latitudes, so does temperature fall as you climb a mountain. Altitudinal migration is an underappreciated yet common type of migration.

One can find information on most aspects of a bird species by consulting The Birds of North America. This work is a series of species accounts, each written by an expert or experts on that species, reviewing all that is known about a particular species. In a 2017 paper, Alice Boyle scoured the 603 species accounts for North American breeders in search of evidence for altitudinal migration. She found that altitudinal migration was reported for 163 species (27%).

As you might imagine, most of these species occur in the western part of North America where many of the Rocky Mountains, Cascade Mountains and Sierra Nevadas are over twice as high as the tallest mountains we have.

Nevertheless, we do have altitudinal migrants in Maine. Here is a partial list: hairy woodpecker, pileated woodpecker, northern flicker, Canada jay, brown creeper, house wren, dark-eyed junco, purple finch and evening grosbeak. These birds may only migrate a few miles as measured along the surface of the earth.

Just to add to the complexity, some species show partial altitudinal migration with some species sticking out the winter at high altitude and others descending where temperatures are more moderate.

What about really tall mountains? Using eBird records, researchers found that 70% of the 302 species that breed at high altitude in the Himalayas engaged in altitudinal migration. Who’s surprised?

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu

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