The red-bellied woodpecker is among the species that have been expanding their range northward in Maine, according to research being conducted for the Maine Breeding Bird Atlas. AP photo

Migratory breeding birds, we hardly knew ye! How quickly the Maine summer passes. Well over 100 species of birds that winter to our south returned to nest in our fair state in the spring. For most of these species, their nesting is done and the fall migration is beginning.

Most of our swallows are gone now. Any you see are likely birds that bred to our north and are following on the wings of our breeding birds to their wintering grounds.

The relatively brief breeding season causes us to reflect on how we sort birds. Black-capped chickadees, American crows and downy woodpeckers live here year-round. We can call them resident birds. But migratory breeding birds spend only about three months in Maine. They are found on their wintering grounds for at least six months. The remainder of their year is spent in migration.

So, ruby-throated hummingbirds are really Central American birds that come to North America to nest. Yellow-rumped warblers are really birds of the southeastern U.S., where they spend the winter. In other words, our migratory breeding birds don’t meet the six-month residency requirement to qualify as a Maine resident.

The end of the breeding season signals the end of the season for the Maine Breeding Bird Atlas. We have just completed the third year of field work in this five-year project.

The state has been divided up into nearly 5,000 blocks, each about nine square miles in area. About 1,000 of these blocks are designated as “priority blocks.” At a bare minimum, Atlas participants will devote 15-20 hours to each of these blocks to ensure thorough coverage of the state.

The Maine Breeding Bird Atlas organizers set a goal of having 30 percent of the priority blocks completed by the end of this breeding season. Happily, the hundreds of volunteers came through; 32 percent of the priority blocks are now complete. Most of the remaining priority blocks have received some coverage already so the goal of completing all of the remaining priority blocks is a reasonable goal. Of course, the dream is to complete all of the blocks.

Maine was one of the first states to undertake a Breeding Bird Atlas. Fieldwork was done from 1978 through 1983. This project was less ambitious than the current atlas with a total budget of less than $10,000. The blocks were six times the area used in the current atlas, so the first atlas gives a coarser view of the distribution of Maine breeding birds. You can download a copy at: bit.ly/3fJmLmw

The first atlas provides a baseline for assessing the changes in bird distribution. It’s possible to see the current maps at ebird.org/atlasme/home. From the home page, click Explore and then enter a species in the Species Map box. Blocks with evidence of breeding are color-coded. You can zoom in and out and drag the map to re-center it.

We know that a number of species have been expanding their range northward in Maine. These range expansions may be driven by global climate change, habitat changes to our south or population increases forcing dispersal.

With the early atlas in hand and the current atlas available on eBird, let’s make a couple of comparisons. In the first atlas, tufted titmouse appeared in only 13 of the blocks, mostly in York and Cumberland Counties. What a difference 40 years makes. The current atlas shows titmice broadly nesting in the lower half of our state with breeding records as far north as Caratunk and Dover-Foxcroft. There are even titmice in the mountains around Rangeley.

Similar effects can be seen for red-bellied woodpecker, house finch and northern cardinal.

The pattern for some species is alarming. In the first atlas, purple martins nested in a broad swath from York County northeastward to the Princeton region in Washington County. Now, we see a striking reduction in martin colonies with only nine blocks reporting active nest colonies.

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