Ever wonder about the migration of birds around here?

This is a Fox Sparrow.

Photo by Liam Hutcheson

Spring’s dawn chorus has gone quiet. The birds’ eggs have hatched, and most of the young have fledged and are on their own. The season has changed, from breeding to migration.

Some of our Thurston County birds spend their whole lives right here, but most migrate to other places as part of their annual cycle. This includes breeding birds that arrive here to take advantage of summer’s food abundance (for example, the swallows and flycatchers) and those that breed elsewhere and spend the winter here (ducks and geese, for example).

Bird migration has puzzled us humans for thousands of years. Where do they go and how do they manage to come back to the same area? The Greek philosopher Aristotle and others thought it possible that birds changed from one species to another from summer to winter, or possibly that birds were hibernating when they were no longer being seen.

Ornithologists and other researchers over generations have studied the migration puzzle and, while we still don’t have all the answers, we now know more about bird migration than the ancient Greeks did.

What makes birds migrate?

Migration’s primary driving force is food. Ponds in northern Canada make good breeding areas for grebes and loons, but winter food is much more available in the open waters of Puget Sound. Migration is difficult and risky, but ensuring a dependable food supply makes the effort worthwhile.

How do the birds “know” it’s time to move? Studies suggest that the timing of long-distance migration is determined primarily by the changes in day length. Local migrations may also be driven by changes in temperature and food supply. Examples of the latter are Fox Sparrows (see photo) and Varied Thrushes. In late Spring, they migrate to higher elevations in the mountains for breeding; however, about the time one realizes they aren’t around, they return from this brief breeding sojourn to spend the fall and winter here, closer to sea level.  

Birds migrate in different patterns. Most fly north-south in flocks along well-established migration corridors. Currently, you might see small flocks of swallows gathered on utility lines. Soon these small flocks will merge into larger flocks and fly south to the tropical Americas. At about the same time, large flocks of waterfowl will be arriving in our area for the winter. (Of course, the same north-south migration occurs in the reverse direction each spring, but for the moment we’re thinking about fall migration).

How do they know where to go?

But how do they know which direction to go? For a flock, it’s easy to imagine that older birds “know” the direction and pass this information on to the next generation. But not all birds migrate in flocks. Some, for example, our little Rufous Hummingbirds, migrate as individuals.

The answer, in part, is that birds can detect magnetic fields. Imagine that you could feel magnetic effects (the magnetic pull of north for example) the same way you can feel hot or cold. Many birds can, and they use this ability, plus the direction of the sun, to accomplish amazing migration feats.

The Bristle-thighed Curlew (see photo) is a bird species that breeds in Alaska and migrates south across the open water of the Pacific Ocean to Hawaii and islands further south for the winter, sometimes flying 2,500+ miles non-stop. That’s amazing, but more amazing is that the adults depart first, leaving the young to migrate alone! Obviously, there is some remarkable innate mechanism at work here, and we still don’t fully understand it.

Bird migration is a major subject of ornithology study. Early work was conducted on homing pigeons (which are actually our familiar Rock Doves) and subsequent studies have involved many bird species around the world. The consensus is that birds use a variety of cues, singly or in combination, to navigate migration. These include magnetism, the direction and angle of the sun, the stars and other visual landmarks that are held in a mental map. This suggests that there might be a lot of mystery and subtlety in what people so often disparage as a “bird brain.”

George Walter is the environmental program manager at the Nisqually Indian Tribe’s natural resources department; he also has a 40+ year interest in bird watching. He may be reached at george@theJOLTnews.com

Photos for this column are provided by Liam Hutcheson, a 15-year-old Olympia area birder and avid photographer.