Turkey vultures have already arrived in southern Maine. John Patriquin/Staff Photographer

Several readers have written to me about the presence of eastern bluebirds at their feeders in the past few weeks. This morning when I was excavating my car from the recent snowstorm, I heard the ringing teakettle-teakettle-teakettle song of a Carolina wren. We’ll return to these two species at the end of this column.

Despite the deep snow cover over most of the state thanks to the March 4 snowstorm, spring bird migration has already commenced. Turkey vultures and red-winged blackbirds have arrived in the southern half of the state. Before the month is over, we can expect common grackles, American woodcock and eastern phoebes.

With the help of over 400 birders, I developed a web tool to allow a user to find when species are expected to arrive back in Maine.

The spring migration has a sense of urgency about it. Reproductive hormones in birds are rising and finding a mate and nesting are foremost on their minds. Since females generally choose their mates, males usually arrive on the breeding grounds first and try to acquire a territory, with the quality often an important criterion in female choice.

The males are on the horns of a dilemma. Each male must consider the tradeoffs of an early or late arrival. An early arrival gets the pick of good territories at the risk of diminished health or even death from lack of food or severe weather. A later arrival trades off the benefit of arriving when food is more abundant and the weather is milder at the expense of having to settle for an inferior territory.

We have over 100 species of breeding birds in the state that are migratory. These species have a range of diets. Seedeaters can afford to arrive early, but insect-eaters like warblers and vireos must wait until leaf-out. But every male is faced with the decision of the optimal time to get to its breeding ground.

Have you noticed that the dark-eyed juncos you see at your feeders during the winter are mostly the striking gray and white males. Females migrate further south for the winter and juveniles even further south. The need for males to arrive early on their breeding grounds means more successful males will tough out a more severe winter with the benefit of being closer to their breeding grounds where they will compete for territories.

Female juncos can arrive later, so they spend the winter in a more hospitable clime. The young birds, less experienced in finding food in the winter and with limited breeding success in their first reproductive season, are found further away yet.

American robin numbers are building in Maine. Robins have the reputation of being a sign of spring. However, the robins we are seeing are arriving from Canada, where they have depleted the fruit in areas to our north. These robins are still doing their fall migration as spring migration begins for red-winged blackbirds and common grackles.

Now back to the eastern bluebirds and Carolina wren mentioned above. These birds are almost certainly not spring migrants but rather birds that have overwintered in Maine. Perhaps some of the bluebirds have pushed down from Canada like the robins.

In the 1990s, the bluebird totals were in the single digits and in double digits in the 2000s, according to the Maine Christmas Bird Counts. Totals were in the hundreds in the 2010s and broke the 1,000 mark in 2021. It is not surprising now to see winter bluebirds in Maine.

Carolina wrens show a similar pattern with single digits on counts from 1990 to 2005, then double digits until 2019 and more than 100 in 2020 and 2021. Perhaps, because of moderating climate, these birds have made the tradeoff of not migrating and being on their nesting grounds early for the challenges of surviving a Maine winter.

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