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—to find good spots for breeding in the spring and to find ample food when seasonally available food runs low. 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.
Sometimes, the resources in your breeding area differ greatly from season to season. When this pattern is highly predictable, you have to leave or starve. No two ways about it. Most birds in this situation, such as those breeding in the northern areas of the temperate zone such as ours, head southward for the winter to ensure that they have ample food.
Why not migrate?
Although most species of birds migrate, at least to some extent, not all do. Why not just flap on down to the tropics for the winter? Sounds pretty good to me. If you’re in an area where all necessary resources—water, food, shelter—are predictably available year-round, you’ll likely stick around all year. In our area, most chickadees, titmice, nuthatches, 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.
Birds that live year-round in the tropics have annual adult survival rates of about 80-90%. That means that if you never leave the tropics, you have odds as high as 9 to 1 that you’ll be alive one year from now. Lucky you. 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.
So why don’t all birds just live in the tropics and never leave? Migration truly is a risky prospect. You expend vast amounts of energy traveling sometimes thousands of miles often through miserable weather (e.g., cold fronts, hurricanes) to get to your breeding grounds or back to your wintering grounds. Food can be hard to find and you’ll need a lot of it. Rest is even harder to come by, especially if you hit bad weather or you have to fly over large bodies of water (e.g., the Gulf of Mexico) with no place to land. When you get there, exhausted and starving, you immediately have to start setting up a new territory. Yet the year-round residents may have already staked out the best territories before you get there—and they’ll fight tooth and nail (beak and claw?) to keep you away from their area and resources. And you’ll have to make this grueling, life-endangering journey twice a year. Somehow, it doesn’t seem quite the idyllic situation it originally did.
Reasons to not migrate seem pretty compelling. So again—why migrate? Although they have higher annual survival rates, tropical year-round residents have much lower reproductive success rates than do birds that breed in temperate areas, whether they migrate north or are full-time temperate-zone residents. So they raise fewer young each year than do those that breed in the temperate zone—in large part because nesting areas are in shorter supply in the tropics. If you live in a temperate zone area such as ours 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 likely raise more young than any full-time tropical resident will. Ya pays yer money and takes yer chances…
In central Colorado, species that depend on insects for a major part of their diets face dwindling resources at the end of each breeding season, as the populations of insects decrease. Nearly all individuals of these species—e.g., swallows, flycatchers, nighthawks, warblers—will leave before foods resources reach dangerously low levels. Birds that rely on flower nectar, such as hummingbirds, leave as the wildflowers die. Waterfowl—such as this month’s pictured bird, the Snow Goose—and shorebirds that need open water for food migrate before ponds, lakes, and rivers freeze. Although it’s a strategy fraught with perils, these birds really don’t have a choice.
When does migration take place?
Surprisingly, fall migration actually starts as early as July for some species, such as shorebirds and hummingbirds. Triggers to head south include changes in day length, lower temperatures, drops in natural food supplies, and hard-wired genetic predispositions. Most species of songbirds begin to migrate in mid- to late September or October; and most migrants have left by mid-November.
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. Other diurnal migrants include woodpeckers, kingbirds, crows, bluebirds, robins, blackbirds, and finches.
But you might be surprised that 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 structure of the atmosphere at night tends to be more stable, creating conditions more conducive to flight by slow-flying birds. The generally 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. Some stay put here, betting that their environments to provide sufficient food and shelter to be able to make it through the winter. Some move just a short-distance—say, from the mountains down to our area; Townsend’s Solitaires, Cassin’s Finches, and Dark-eyed Juncos fall into this category. Others, including Mountain Bluebirds and some of our hummingbirds, move south several states, to winter in Texas or Louisiana. And still others, such as warblers and flycatchers, cover thousands of miles birds are currently on the move.
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 breeding season disappears in our rearview mirror.