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.

Time to Go - Regardless

During the second week of August, it was time to go. Regardless.

When we lived in Pennsylvania, the House Wrens arrived during the last week of April. Their bubbly, unending song began before first light and ended after last light. Six wren houses were scattered around the yard, and they used them all. Some only had a few sticks in them, but at least two, sometimes three, were crammed tight with sticks, except for the small cup that formed the nest.

They seemed to choose the box immediately outside our bedroom window for their first nest, which put their song very close to us for the first couple of months. Wrens volubly proclaim their every movement. And then the first nest would be silent. The young fledged and disappeared into the shrubbery and moved off toward denser habitat than that provided by the yard.

The activity shifted toward a second box, as did the voluble song. The second clutch of eggs had been laid even before the first brood had fledged, the female incubating the former, the male feeding the latter.

Many summers this scenario occurred a third time. This time the nest box was in the cherry tree at the side of the house, near the drive. The activity was in full gear in late July and early August, the wren singing before every entry into the box, and again with every departure, and often from somewhere in the yard as he foraged for the young in the nest.

Then during the second week of August, it was quiet. One day, I would be aware that the background noise of the summer was gone. Most years I could tell you the date when they first arrived in April, because I recorded such arrivals. But departures are more difficult to record. Unless you take attendance every day, you are not aware of an absence. You don’t write down last observations, because you’re never sure they are the last. One day I realized that the wrens were gone. The realization came during the second week of August. The wrens had left. I would not hear them again until the following April.

Sometime after the wrens had departed, I gathered the wren boxes from around the yard, and cleaned them out before bringing them in for the winter. It was always the box from the cherry tree, the one that the wrens had been using just before they were suddenly no longer there. One year as I pulled out the dense stick nest, I found five unhatched eggs. Another year, I found four hatchlings and an unhatched egg. Most often I found four to six nestlings, perhaps half way to fledgling status. None were alive.

The nest had been abandoned and the eggs or hatchlings or nestlings were left to die. The internal clock of the wrens told them it was time to go. Regardless. Migration must begin. The wrens departed. For three and a half months, parental activity had been frenetic. The instinct to breed and raise young drove their every waking moment. Occasionally they must have consumed some insect or grub of their own, but these were quick bites squeezed in among the hundreds of trips they made to feed first one brood, then a second brood, then a third.

And then a second instinct took control, a more primal instinct than the parental one. Individual survival trumped the survival of their offspring. It was time to go, and they went. Quietly they headed to the thickets and woods, in a general southerly direction. They bulked up, adding fat for their long journey. The young in the nest were left to die.

Nature is a harsh odds maker, and it plays the odds. The odds of any fledgling bird surviving to the next year and being able to breed are long. Some estimate the odds to be, at best, one in five. The odds of an adult bird who has bred this summer surviving to breed again next summer are better. One in two is often suggested, sometimes a little better. The purpose for any individual is to pass along its genes. And that means playing the odds. Small parent birds expend tremendous energy and effort to raise their young, but rarely do they deliberately endanger themselves in the process. To do so would mean an end to their genetic line - a dead end.

Small birds defend their nests against predators such as larger birds, or snakes. But they don’t sacrifice themselves; that would be a literal dead end for themselves and their offspring. The young they raise may not survive, but if the adults survive they may have another opportunity to breed in the future.

I have used the House Wren as an illustration and example. There are lots of exceptions to the generalizations I have drawn from the wrens. There are birds that stay together in family groups throughout the year. There are young that stay with parents as helper birds during the next breeding season. But for most small birds, there is a harsh limit to parental care. When the limit is reached, the parents focus their energy only on their own survival.

We often romanticize and sentimentalize nature. We are entertained, amused, and delighted by the birds which flock to our feeders, by their cuteness, vivacity, acrobatics, and antics. Sometimes we are upset when some seem aggressive, act like bullies, seem voraciously greedy. We scarcely recognize that none of them are influenced by human values.

I watch harried parent birds feeding their young. The young flutter and call and beg, and chase after their parents to be fed. I smile and think about how glad I am that my children are raised, and feel a moment of sympathy for those parent birds. But then I watch more closely, and I see that the parents are leading their young to food, showing them where to find food. Often the young birds that are slow to learn are driven away by their parents. Dependency by the young must come to an end; they must learn to survive on their own, for their sake, and their parents’ sake. A point comes when the parent must turn to its own survival and let whatever happens to its young, happen.

I felt a poignancy and sadness whenever I cleaned one of those wren houses and found a nest of dead babies. But then I reminded myself that even if those young had fledged, most would not have survived when finally left on their own. For the parents birds, it was time to head south, even though there were young still in the nest. Time to go. Regardless.