September 20, 2018
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Bird migration a combination of order and chaos

Bob Duchesne | BDN
Bob Duchesne | BDN
Sometime in mid-September, a stream of broad-winged hawks, like this one, will join sharp-shinned hawks, American kestrels as they begin to migrate and fly past Maine.
By Bob Duchesne, Special to the BDN

Birds migrate. This fact is universally understood, yet universally underappreciated. The science actually gets pretty complicated. Take this week, for example. Some birds started heading south weeks ago. When was the last time you saw a tree swallow? The remaining birds didn’t migrate this week. Why not?

Maine has enjoyed southwesterly winds all week. It’s been unseasonably warm. While some birds are getting itchy to migrate, they’re smart enough to know that flying into a headwind is not advisable. Better to wait. Soon enough, the winds will change. When that happens, all heck will break loose.

Migration is neither a smoothly running event, nor a thoughtless occurrence. It’s a fascinating combination of order and chaos. The birds can read the signs in nature, and with a little practice, so can you.

For example, hawk migration has begun. It will peak in about two weeks. Sometime in mid-September, a stream of broad-winged hawks, sharp-shinned hawks and American kestrels will fly past Maine’s prominent coastal mountains. Expert birders will watch the weather maps, knowing that sometime during that period, a low front accompanied by lousy weather will pass over the state. It will be followed by rising air pressure and a northwest breeze. On that day, hawk migration will spike. Hundreds of raptors will soar past Maine’s more famous hawk-watching sites, including Cadillac and Beech Mountains in Acadia National Park, Blue Hill in Blue Hill, Mt. Megunticook in Camden, Bradbury Mountain in Pownal and Mt. Agamenticus in York. The hawks have been waiting for such a breeze, and it is going to be a lively morning.

Most songbirds migrate at night. The air is cooler, the wind is kinder and the threat of predation is less. We know that daytime migrants use landmarks to aid navigation. Geese and ducks follow rivers. Raptors follow ridgelines, rising on the updrafts. Many birds navigate by the sun, instinctively knowing where the sun ought to be at certain times of day. But neither landmarks nor sun position are available for birds navigating in the dark. So what’s going on up there?

Many migrants have built-in compasses — small crystals of magnetite in their nostril areas, allowing them to sense the Earth’s magnetic field. Night flyers also have the ability to determine direction based on the North Star. Researchers subjected caged birds to alternative night sky patterns and discovered that the birds were not using the star patterns as a map. Rather, they were able to sense the rotation of stars around the sky and figure out the center point of the rotation, which coincides with the location of the North Star.

Some birds have exceptional homing ability. Homing pigeons may be the most famous, able to mentally map their surroundings and use clues to find their way home. In one experiment, homing pigeons were released over the ocean. With no landmarks to guide them, only 2 percent made it home. In another experiment, pigeons were fitted with opaque contact lenses. Most did find their way home, indicating that they likely use their other senses to detect familiar odors and sounds by which they navigate.

The Manx shearwater is an ocean bird with great homing ability. In one experiment, shearwaters were removed from their burrows on one side of the Atlantic and flown by commercial jet to the other side. Although the birds had no knowledge of the route that led them astray, nor any landmarks to help them return, they were home again within 12 days.

OK, but what about that songbird aloft on a cloudy night with no stars to guide it? There is experimental evidence to show that some birds fix the position of the setting sun and then use that fix to point them the right way after dark. Some birds appear to use prevailing winds to establish a directional fix. But winds are fickle, so birds can get disoriented and fly off-course until sunrise allows them to reorient.

For that reason, many of our songbirds prefer to wait for a favorable night to depart. In this week of southwesterly breezes, it’s a good bet that most haven’t gone anywhere yet. On the next clear night with a fresh breeze from the northwest, a torrent of birds will take flight at dusk. On a still night, you can actually hear them passing over. Many birds have distinctive flight notes that penetrate the darkness, and alert each other to their whereabouts. In Maine, you can not only see migration happen, you can hear it.

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.

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