A tree swallow. Credit: Courtesy of Bob Duchesne

By the time you finish reading this, more swallows will have poured into the state. I hope.

Tree swallows began to arrive around Easter. They nest across the upper latitudes of North America from Labrador to Alaska, with a summer range that extends below the Mason-Dixon line. They winter in Florida, the coastal edges around the Gulf of Mexico, and throughout the Caribbean and Central America. It’s likely that the earliest tree swallows showing up in Maine never left the country.

Northern rough-winged swallows began to arrive right behind the tree swallows. These swallows nest in all of the Lower 48 states, but barely range into Canada. Most winter in Central America, though some spend the cold months in southern Florida and along the Gulf Coast. As with tree swallows, the birds wintering in the southern U.S. enjoy a relatively short migration back to Maine.

Barn swallows vacate our continent in winter. When they migrate south in autumn, they don’t stop until they get at least as far as southern Mexico. It takes them a while longer to get back in the spring.

Cliff swallows don’t stop winging their way southward in autumn until they get all the way to South America. Naturally, it takes them longer to make their way home over the longer distance. Furthermore, they’re in no rush. They migrate by day, foraging leisurely as they go. None have yet been seen in Maine this year. They should begin to arrive in about a week, with most turning up in early May.

Bank swallows outmigrate the cliff swallows, some wintering as far south as Argentina. What, the winter bugs aren’t plentiful enough in Central America? Figuring out why some birds go where they go is hard. But knowing how long it takes them to get back is easy. The farther the trip, the longer the journey. We’ll see you guys in another couple of weeks. I hope.

The problem is that swallows are aerial insectivores. Like many other songbirds, they eat bugs. However, while most warblers are adept at picking food off leaves and bark, swallows need to snatch most of their food in flight. Few birds are more agile in the air.

Years ago, I watched a merlin make a surprise attack on the nesting bank swallows behind the post office in Medway. The swallows easily dodged the assault, and then angrily chased the medium-sized falcon to the horizon.

I don’t know if the bank swallows are still nesting back there. Or anywhere. They’re in the process of being added to the state’s list of threatened species, along with the cliff swallow. I used to know a dozen places to find nesting bank swallows. I don’t know a single one now. The current population of bank swallows in Maine is only 1 percent of what it was in 1966, when biologists started tracking the decline of songbirds.

Similarly, cliff swallows are disappearing at an alarming rate. They used to nest under every bridge, a few highway overpasses, several buildings on the University of Maine campus, and a considerable number of grange halls in the area. As of last year, I could still spot a few nests under the Veterans Remembrance Bridge in Bangor. That’s about it.

Other swallows are declining as well. Tree swallows are down 30 percent. Barn swallows down 25 percent. Northern rough-winged swallows down 18 percent. There are no officially verified reasons for the declines, but the usual suspects are habitat loss and food shortages. Other aerial insectivores have also declined precipitously, suggesting a major reduction in flying insects.

Habitat losses contribute, certainly. Tree swallows have lost ground to competition from other cavity nesting birds, such as house sparrows. Happily, they’ve regained some of their numbers as we’ve put up more bird houses. Although many of those boxes were put up to help eastern bluebirds bounce back, tree swallows have also benefited.

Barn swallows have fewer barns and abandoned buildings to nest in. Improved flood control has reduced the available habitat along riverbanks for bank swallows. And for all swallows, there may be threats on their wintering grounds in Central America and South America that we barely comprehend.

Regardless, swallows are cool. Some songbirds lurk in the shadows. Others hide in the treetops. Many are just shy. But swallows live right out in the open, not much perturbed by human presence, acrobatically foraging over ponds, fields, parks and golf courses. They chatter giddily while on the wing. Right now, they’re coming home, arriving daily. We’ll miss them when they’re gone.

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Bob Duchesne, Good Birding

Bob Duchesne serves as vice president of Maine Audubon’s Penobscot Valley Chapter. He developed the Maine Birding Trail, with information at mainebirdingtrail.com. He can be reached at duchesne@midmaine.com.