Saturday, October 04, 2008

Why Do Birds Migrate?

During September, Putney Mountain attracts day hikers, picnickers, leaf peepers, nature lovers, lovers, photographers, and many others seeking some experience of the out-of-doors during the waning days of summer. Many are curious about the hawk watchers who stare for hours at the sky, wondering what they are doing and what they are seeing. There is a lot of on the spot education about hawks and migration which takes place.

Sometimes the education is planned, as when a school does a field trip to Putney Mountain. Some field trips are well planned outdoor classrooms; others appear to be a day out of the classroom. For as long as I have been trekking to the ridge to watch hawks, I have seen someone among the day’s hawk watchers ready to do an instant lesson for school groups on migrating hawks, solicit the help of their sharp young eyes, and convey an appreciation for the wonders of the natural world.

Recently I did one of those spur of the moment outdoor classrooms, explaining such rudiments of hawk migration as where they are going, and how they use the thermals and updrafts along the ridge to help them get there. Then I asked, “Why do the hawks migrate?”

“To get away from the snow,” a student said. “Partly true,” I replied, thinking to myself that the answer is definitely true for those thin blooded retirees who head south for the winter. “What other reason?” I asked.

“Food,” said a youngster wearing binoculars.

“Right!” I replied, and now he also wore a big smile. I gave an example. A Sharp-shinned Hawk had just made a very fleeting appearance as it dove into the valley. That Sharp-shinned Hawk was chasing a small bird, because that is what a sharpie eats - small birds. The Sharp-shinned has a long tail and short rounded wings that enables it to twist and turn through the forest as it pursues its food. Most small birds migrate south for the winter. If most of the food you eat flies south, then you also fly south. And so, the Sharp-shinned Hawks are migrating south.

Generalizations are always risky, and almost never completely true. To say that “food” is the reason birds migrate is a risky generalization. Even so, most migrations by most birds are somehow related to food.

Bird migration is an exceptionally complex natural phenomenon which science has only recently begun to unravel. The how, why, when, and where of migration is different for nearly every species, but in some way food is almost always involved.

Most of our favorite songbirds are not really “our” birds. They are neotropical birds. They fly north to take advantage of our temperate climate. Our temperate climate produces abundant protein during a relatively short growing season. Those annoying mosquitos and plant eating bugs in the garden are a bountiful larder for birds needing lots of nourishment for their young. Some warblers arrive on their breeding ground at the end May. By the end of July, they have hatched and fledged their young. They don’t linger to enjoy Vermont in August; they start south.

The same is true for many shorebirds. They nest in the Arctic, where the growing season is even briefer and where food protein is even more intense. They breed; they incubate; the eggs hatch. By the first week of July, the adults of some species are already heading south. The precocial young feed themselves, and a month or two later, guided by their genetic encoding, the hatch year birds migrate south. Their food in the Arctic will soon be gone.

Cold weather and snow are not, by themselves, the reason that birds migrate. There are many species that live in Vermont year round, even when the temperatures plunge or the snow pack is measured in feet. Their ability to survive depends upon finding enough food. If food is scarce, some of these species will migrate further south. Some of the weaker individuals will die.

Bird feeders enable some birds to spend the winter. Species which have extended their range northward, such as the cardinal and titmouse, have been assisted by bird feeders. A few Sharp-shinned and Cooper’s Hawks have discovered that they do not need to make a long, energy expensive migration. Instead, they can grocery shop during the winter from bird feeder to bird feeder.

Last year, bird watchers in New England were treated with an exceptional number of boreal species: redpolls, shrike, Pine Grosbeak, Bohemian Waxwing. Pine Siskins appeared at my feeders in early September last year and were common through the winter. This year, Pine Siskins are absent and it is likely they will remain absent or very rare throughout the winter.

It has to do with food. The availability of the various food resources that the siskin and other boreal species prefer during the winter vary from year to year. A researcher in Ontario gathers reports from throughout Canada and forecasts whether and where such species may appear. For example, the Pine Grosbeak is a mountain-ash berry specialist in winter. He predicts that Pine Grosbeaks will stay north of most birders this winter because mountain-ash berries are abundant in northern Ontario.

For the last several weeks, when there haven’t been hawks to watch on Putney Mountain, I have been watching Cedar Waxwings. On some days, several dozen would be flying out of the tree tops, flycatching through hordes of insects. As the cold weather comes, those waxwings will form into large flocks and shift their diet to berries. They will migrate about the countryside and towns, pausing for a while to feast on the fruit, then do another short distance migration to another berry rich area.

The short answer to why birds migrate is “Food.” Migration from mountain top to valley. From neighborhood to neighborhood. From boreal forest to temperate forest. From temperate forest to tropical forest. From prairie pothole to Gulf Coast. From northern lake to Atlantic coast. From North Atlantic to South Atlantic. From New England to the deep South. Somewhere in the species specific complexity of those various short, medium, or long distance migrations, there is a food component. The energy expense and dangers of migration is offset by the availability of food for survival or reproductive success.

Other times on Putney Mountain, I have pursued sparrows, towhees, and warblers as they have moved through the thick brush, feeding on the abundant berry crop. Occasionally they pause to give me a glimpse, and then I can see the feathers around their beaks stained with berry juice. They are feeding up so they can continue their journey south. It makes for good birding.

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