Laura Erickson's For the Birds

How and Why Birds Migrate

(Recording of a White-throated Sparrow)

Probably the most fascinating area in all bird study is migration. From ancient times people have wondered about the mystery of migration. Of course, before world travel was common, people didn’t realize that birds were actually going from one place on earth to another. Ancient Greeks believed that some birds spent their winters on the moon, which apparently had some bird seed embedded into the green cheese. The Greeks also believed that birds like swallows buried themselves in the mud each fall. Sure enough, when spring came, the swallows always showed up near muddy ponds first, and never appeared over larger lakes until the ice was gone. What more proof could you ask for?

Of course it’s fun to look at the knowledge and beliefs of earlier times and think how far we have advanced since then. But we clever 80’s people don’t know as much as we’d like to think we do. In another century, if we haven’t clevered ourselves out of existence, our great grandchildren will look back on these days and think how little we understood about migration.

The first real breakthrough in our understanding of migration came about as a by-product of the Renaissance. European explorers discovered some of their favorite birds in Africa and Asia, and slowly the idea dawned on them that the birds might actually be more experienced travelers than they. After that breakthrough, the main focus of migration studies was to determine the breeding and wintering ranges of species.

The next real breakthrough in migration studies came much more recently–in 1969–when an ornithologist named William Keeton made a hypothesis that birds literally have a sixth sense – one that goes beyond sight, hearing, smell, taste, and touch. Birds also sense the magnetic pull of the earth. The way Keeton tested his hypothesis is one of the most elegant examples of the scientific method you could want. He fitted little metal bars on the backs of homing pigeons. Half the birds got magnetic bars, and the other half got bars that were the same size and weight, but were made of non-magnetic brass. When he released the birds some distance from the home loft on overcast days, five out of seven birds with brass bars oriented correctly and returned to the loft; the birds with magnetic bars scattered randomly. But when he released the birds on a sunny day, all the birds oriented correctly. Later studies showed that the brains of some birds contain crystals of magnetite.

Now we know that pigeons get their bearings from both the sun and the earth’s magnetism. We also know that some night migrants use the stars. Experiments at planetariums have shown that birds concentrate at the side of a planetarium where the projected stars point north in spring, even if that’s far from true north. Some some birds use landmarks to find their way–especially geese and other birds that lead long lives and don’t need to migrate over large bodies of water. On the other hand, the Ruby-throated Hummingbird, has to fly nonstop over the Gulf of Mexico every spring and fall–it would be in big trouble if it didn’t have an internal, instinctive system of navigation. Adult hummers leave the breeding grounds before the young each fall, and yet the inexperienced hummingbirds find their way on their own.

I’m glad migration is still a mystery. The more we know, the richer our imaginations can grow with questions about how such tiny creatures can accomplish feats that we humans can only dream of. And not all the knowledge in the world can diminish the feeling we get when we hear our first White-throated Sparrow of the spring.

(Recording of a White-throated Sparrow)

This is Laura Erickson, and this program has been “For the Birds.”