The changing seasons

A barn swallow

A barn swallow

Let’s say you’re a bird. Your breeding area this spring and summer abounded with budding plants, fresh fruit, delicious insects—plenty for you and the kids you raised. But what about the coming fall and winter? Should you stick around or should you head for an area that might offer a better menu this time of year? It all depends on what your environment has in store for you. Birds migrate for 2 primary reasons. In early spring, the press is to find good breeding areas. But as summer draws to a close, the availability of natural food sources reigns supreme.

In our area, most chickadees, titmice, nuthatches, crows, jays, and woodpeckers can count on adequate reserves of seeds, berries, and dormant insects to see them through the winter. So they try to tough it out through the winter. However, most species migrate to warmer climes, at least to some extent. Why don’t all birds migrate?

Migration truly is a risky prospect. You expend vast amounts of energy traveling sometimes thousands of miles. Food may be hard to find; and rest can be even harder to come by, especially if you have to fly over, say, the Gulf of Mexico, with no place to land. And you’ll have to make this grueling, life-endangering journey twice a year. Birds that head north for breeding season, then south again for the winter, have annual adult survival rates of about 50%—meaning that only about half of the adults that migrate north in the spring will still be alive the next spring. Migration can be the death of you, quite literally.

But if you live in a temperate zone area such as Colorado all the time, you have an even lower annual survival rate, in the range of 20-50%. Winters here can—and, odds are, likely will—kill you. But if you survive to the next spring, you’ll have the first shot at the best breeding areas, before the migrants arrive. You’ll also likely raise more young that season than even a full-time tropical resident will. Ya pays yer money and takes yer chances…

When folks think of migration, many picture Canada Geese in a V formation, winging noisily to the south in the daytime sky. Indeed, some birds do migrate during day. In addition to geese, soaring birds such as hawks, cranes, and storks rely on sun-heated thermals for lift and must migrate when the sun warms the air. Swifts and swallows, which feed as they fly, also have to migrate by day.

But the majority of species migrate almost exclusively at night. Migrating at night offers a number of advantages. Birds have more time during the day for feeding and replenishing fat stores. The atmosphere at night is more stable, better for slow-flying birds. The cooler air at night reduces stress for the birds from heat and dehydration.

Yet birds don’t have to depend on moonlight for nocturnal flight. Avian navigation at night uses a variety of skills, including using the stars, sensing changes in the earth’s magnetic field, and perhaps even smell for some species! Of course, a full moon offers a bonus for nocturnal migrants—and for humans interested in observing this phenomenon. Aim a pair of binoculars at a full moon during this time of year and you might catch glimpses of the silhouettes of songbirds, quietly passing the face of the moon—rather like E.T. on Elliott’s bicycle.

Fall migration is ramping up now as species head out to find warmer areas for winter. In The Edge of the Sea, Rachel Carson wrote:

There is symbolic as well as actual beauty in the migration of the birds… There is something infinitely healing in these repeated refrains of nature, the assurance that night after night, dawn comes, and spring after winter.

Sentiments to keep in mind as the breeding season disappears in our rearview mirror.

– by Christina Mitchell

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