North American Migration

Learn more about migration here.

What makes birds migrate?  It’s a fact that birds have an enormous advantage over all other creatures, simply because they have feathered wings.  When food is scarce, they can quickly travel to more abundant sources and easily locate their prey from the air. When winter descends, birds have little trouble relocating to warmer climates and more plentiful food sources. The combination of these abilities and events over a long period of time eventually culminated in large populations of birds leaving their summer grounds en-masse for more hospitable climates when winter approaches.  This is the phenomenon we know as migration.

Some birds never migrate, but for birds that do, it is not choice but a programmed response to environmental cues. In North America there are about 350 migrating species, 250 of which are considered Neo-tropical, meaning that they winter in Mexico, Central and South America as well as the West Indies. It is generally accepted that a change in day length causes significant changes in a migrating bird’s brain. Among other things, this internal change stimulates a bird’s appetite in order to build up an enormous storage of fat, induces a sense of restlessness and encourages the tendency to flock together. There is safety in numbers during this long, stressful journey. It’s an interesting fact that the farther north a bird’s summer range is, where seasonal day length is most pronounced, the more likely it is that the species will migrate. Near the equator, where the days and nights are always of equal length, there is a much lower percentage of migrating birds. The change in daylight in a bird’s winter range triggers additional hormonal changes that also prepare the bird for breeding when its return northward migration is completed.

Migratory birds are different from non-migratory birds in other ways, too. All birds have hollow bones, insulating feathers, and unique “wishbone” which powers their wings with strong pectoral muscles, and lungs which are kept constantly inflated by extra air sacs not found in other creatures. However, migratory birds tend to have much larger pectoral muscles with more blood vessels, which make them much more efficient at producing and using energy. Their wings tend to be longer and more pointed. Even their blood is different. While all birds have high concentrations of red blood cells for efficient oxygen dispersal, some migratory birds actually have two kinds of hemoglobin in their red blood cells that carry oxygen through their bodies in different ways. This is what allows them to fly at high altitudes where the oxygen levels are very low. Another interesting difference about some migrating birds is that their activity period shifts from daytime to nighttime right before they begin to leave. Traveling at night and at high altitudes, birds can take advantage of the lack of predators, a calm air mass and have a lesser chance of overheating or dehydrating. Most songbirds and shorebirds migrate at night.

In North America there are four major migration routes, known as the Atlantic, Mississippi, Central and Pacific Flyways. A flyway can generally be described as a broad geographical area of travel consisting of hundreds of widely diverse, individual migration routes. No two species of birds will travel exactly the same route from beginning to end. As these routes continue further south, they tend to blend and converge. In Panama, all the North American Flyways merge into one and then spread out again throughout South America. Of course, some birds have already reached their winter grounds long before the flyways merge, while others will continue farther south to reach theirs. Flyways tend to follow major geographical landmarks, such as the Atlantic and Pacific coasts, along the Rocky Mountains and the Mississippi River. For the most part, migration routes and the Flyways are along areas with no major mountains or water areas to cross and offer the longest line of sight. This brings us to the next factor in migration; navigation.

Migrating birds navigate with a combination of different methods. Along with sighting visual landmarks, following rivers, coastlines and mountains, birds also have a mineral called magnetite in their brains. It is thought that this mineral enables a bird to monitor the earth’s magnetic fields to guide them north and south. Generally it is known that birds migrating by day also use the sun as a reference point, while birds migrating by night use the stars. And it’s probable that birds follow other flocks. What is not known is how birds compensate for and overcome changing constellations, sunless days, interferences in magnetic fields caused by radio towers and the like. Somehow the birds are able to recalibrate their magnetic compasses based on the visual cues that are available to them at different times.

At some point in the distant past, survival of the fittest determined that some birds were more likely to thrive if they migrated rather than remaining in the same area year round. However, migrating birds encounter many more obstacles now than even a hundred years ago. Among these include loss of nesting habitat along the flyways as grasslands are converted to croplands, loss of coastal resting areas, loss of rainforests in the South Americas, interference by planes, skyscrapers and towers to name just a few. All of this has shown to cause a significant decline in some migratory bird species. However, there is good news. Working from information gathered by the BBS (Bird Breeding Survey) and other sources, the National Fish and Wildlife Foundation began an extensive program in 1991 called Partners in Flight. Involving many organizations on local, state and federal levels, as well as conservation groups, individuals and philanthropic groups, this program is dedicated to the preservation of migrating birds, providing research and millions of dollars to the cause. As a result, they have been instrumental in the protection of avian habitats and strive to improve education, research, and the management of migratory birds.

Many backyard birders find themselves located in a flyway zone or on at least one migratory route. Resting places along these routes are vital to traveling birds. Sometimes they will stop for only a day, but many times birds need several days in their resting location to build up enough fuel for the next leg of the trip. If you wish to lend a helping hand to these birds, it’s easy to provide the basic necessities; food, water, and shelter. Shelter can be ornamental shrubbery or even branches from dead trees piled in a corner of your property. If you have a brook or creek that has open water in the winter time, you won’t need to worry about a water supply, but if not, you might want to consider heated water in the form of a bird bath heater or even a bath that has a built-in heater. Food choices are endless. Seed blends, fruit, suet or even stale baked goods will be well received. If you are lucky enough to live near a flyway or migratory route, spring and fall are very exciting times!

— Roxanne Brune

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  • Susang6 March 23, 2011 at 11:44 pm

    I enjoyed your articles and feel I have learned more about migrating birds.