Bird Migration

By Ann Hingas

Birds use "maps" and "compasses" in migration

Geese are resting in fields throughout the state, and their voices fill the skies in mornings and evenings, and tiny yellow-rump warblers flit through lakeside greenery hunting bugs to fuel their trips south.

The annual bird migration is beginning, and it’s the most dramatic, powerful event in the natural world. It will last for at least a couple of months, and it’s a matter of life or death for these feathered descendants of the dinosaurs.

Some birds make incredible flights. The Arctic tern, for example, is the long distance champion. It flies from the North Pole to the South Pole and back every year.

Just as impressive is the migration route of many hummingbirds. Most of our eastern ruby-throated hummingbirds cross the Gulf of Mexico on their way to wintering areas in Central America. No, they do not ride on the backs of geese. They fly, non-stop, across the Gulf.

These are startling feats of physical endurance. If there were a sanctioning body for birds, they’d all be getting tested for steroid use. But don’t think birds are all brawn and no brains either. Their mental powers, exhibited in their navigation skills, are awesome.

Plenty of humans can’t find their way across town, let alone make it to Costa Rica without a road map and compass. Birds don’t have pockets for maps, and they don’t carry Silva compasses, but they can fly thousands of miles and find their favorite nesting tree, bird house, rose bush, gravel bed or leafy forest floor, year after year.

How do they do it? There’s been a lot of research over the past twenty years trying to answer that question. Here are some of the things research biologists have discovered:

*Some birds have their own "compasses" built into their brains and orient according to magnetic north. In one of the oddest experiments, researchers put little magnetic caps on homing pigeons that reversed magnetic polarity atop the birds’ heads. They flew the exact opposite direction that they would have flown when released.

Since those experiments, molecular biologists have identified a magnetic particle in some bird brain cells. These particles change alignment according to magnetic influence.

And, biologists now suspect that some birds can detect minute electro-magnetic signals at many points along their migration routes, enabling them to zero in on specific locations.

*Celestial navigation is a primary means of navigation in some birds, especially those that migrate at night. The landmark research on this subject involved the indigo bunting. In an simple but elegant experiment, biologists put caged buntings in a room where different night skies were projected onto the ceiling.

The cage bottoms were covered with white paper, and an ink pad was placed in the center. By simply looking at tracks produced as the birds hopped from ink to paper, it was possible to see which direction the birds were orienting under different overhead stars. When showed a fall sky to the buntings, they hopped about in what they thought was the southern part of their cages. Under a spring star map, they reversed their direction.

*Birds use "land maps" that young birds learn on their first migration. Geese are the most obvious map readers. Geese habitually watch the land as they fly, looking for resting places, feeding grounds and water. That they depend on visual map reading is obvious when they get caught in fog.

Where birds that rely on magnetic direction finding can navigate in fog, geese do not. They get off course and land in unlikely places. Geese often mistake wet city streets that are reflecting fog-filtered street lighting for water and land there.

There are probably other means birds use to find their way. Some scientists wonder if some birds use the sense of smell as a map in the way salmon and other fish use chemical clues to find their "home." Others think vibrations from land may play a role.

It would be a mistake to think that any bird uses only one of these methods to navigate. More likely, they use them all or combinations of them in their annual long distance flights.

Here’s how it might work. First, the bird uses either its magnetic compass or celestial navigation skill to get started in the proper direction. It uses these abilities to maintain a general heading. The migrant resorts to its memorized land map to locate specific feeding, resting or nesting areas along the route.

They may then resort to smell and vibration readings to zero in on home, but with their excellent vision and memories, it is reasonable to say that birds know what home looks like when they see it.

Reduced to these terms, it seems pretty simple. It isn’t. Such complicated information handling is human-like in it’s complexity. It is a good demonstration of how genetic and learned behaviors combine to increase birds’ survival chances.

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