It was a rarity to see turkey vultures in Maine 40 years ago. Now there are records of them nesting as far north as the tip of Aroostook County. Denis Cahill/St.Catharines Standard, via AP

This column is the last in a series on changes in Maine bird populations over the past 200 years in honor of the Maine bicentennial. We’ll look at common birds that are increasing and some that are showing troubling declines.

The landscape of Maine has changed greatly over the past 200 years. In 1820, forests were extensive. We reached a maximum of agricultural land at the expense of forest around 1900, and some of this farmland has reverted to forest in the last century.

Birds have specific habitat requirements, so any change in the landscape will benefit some species while harming others. Clear-cutting is bad for red-breasted nuthatches and Swainson’s thrushes, but leads to increases in mourning warblers and Lincoln’s sparrow.

While clear-cutting in Maine had a negative impact for red-breasted nuthatches and Swainson’s thrushes, the impact actually led to an increase in some species, like the Lincoln’s sparrow (pictured). Lynda M. Gonzalez/Dallas Morning News

The Breeding Bird Survey, begun in 1966, is our best tool to gauge the population trajectories of our birds. Some of our most common birds like red-eyed vireos, ovenbirds, northern parulas and black-throated green warblers are showing increases. So too are eastern bluebirds, helped in part by nest boxes erected in proper habitats.

A suite of species have established populations in Maine in the past 40 years or so.  Tufted titmice now nest broadly throughout the southern half of the state, but had the merest foothold in extreme southern Maine in 1980.

Carolina wrens occasionally  appeared in Maine in the last quarter of the 20th century. They have pushed north, nesting as far north as central Maine.

Northern cardinals have been expanding northward as well as blue-winged warblers, prairie warblers and eastern towhees. Red-bellied woodpeckers have gone from an occasional vagrant to a modestly common breeding bird in the southern half of the state.

Turkey vultures have staged the most dramatic invasion we have had over the past 40 years. They have gone from rarities in the state to common breeding birds. We even have nesting records from the northern tip of Aroostook County.

How do we explain these northern range extensions? The answers are complex and likely different for each species, but ameliorating climate certainly plays a role. Populations of these species may be doing particularly well to our south, forcing some individuals to push north into Maine to avoid overcrowding.

We can celebrate a couple of species that have responded well to conservation efforts. Bald eagle and osprey populations plummeted in the 1950s and 1960s because of the impacts of DDT. This insecticide gets concentrated as one moves up a food chain. In birds, high DDT levels interfere with calcium metabolism. Affected females laid eggs with thin shells that were crushed by the weight of an incubating parent. The banning of DDT has allowed populations of these birds to bounce back.

As nice as it is to have new guys on the block, some of our resident birds are not doing well.  We can divide these species into three groups of ecologically similar species.

First are the aerial insectivores, those acrobatic birds that capture insects on the wing. In the nightjar family, common nighthawks and whip-poor-wills are declining. Chimney swift numbers are down. Two of our most common birds, tree swallows and barn swallows, show dramatic population decreases. Purple martins have decreased their breeding range in the state.

Flycatchers are in trouble too. Eastern kingbirds, least flycatchers and eastern wood-pewees show rapid declines.

We know that flying insect abundance is decreasing, providing a reasonable explanation for the decline of these birds. Eastern phoebes are the only serial insectivore that seems stable. Unlike other aerial insectivores, they can feed on fruit when flying insects are not available.

A suite of neotropical migrants, which nest in North America but winter in the tropics, are in trouble on both wintering and breeding grounds. Species in Maine with declining abundance include wood thrush, veery, chestnut-sided warbler, yellow warbler, American redstart and scarlet tanager.

Finally, grassland birds are imperiled. Savannah sparrows, bobolinks and eastern meadowlarks are all declining.

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