by Janis K. Oppelt |
Birds Traverse the Globe Between Summer and Winter Homes
I first saw the Baltimore oriole, our state bird, in April a few years ago in the midst of a rain forest in Belize, Central America. There we were, my husband, Russ, and I, contentedly floating down a tropical river on a boat tour when our naturalist guide pointed him out to us. With binoculars, we could see the markings of the male oriole clearly: solid-black head, back and most of the wings black; broad white wing bars; and those telltale orange shoulders and under-parts.
As I admired this fine, feathered fellow, I realized that in spite of his namesake, I had never seen him in Maryland. Id assumed that Mr. and Mrs. B. Oriole took their temporary winter respites in the tropics, but I discovered differently when I began researching neotropical bird migration. After reading the works of several notable ornithologists, I recently spoke with Bruce Peterjohn, a wildlife biologist for the U.S. Geological Survey (USGS) Wildlife Research Center in Laurel, Maryland. Baltimore Orioles only spend about two to three months a year on their northern breeding grounds, he explained. Between August and April, they cross the southern U.S. border to their winter [non-breeding] grounds.1
What a surprise to learn that when our avian representatives, and other migrants, arrive in the spring, they are just visiting, not necessarily coming home. They stay only long enough to find mates, procreate, and raise a family. This confirmed much of what Id read while delving into the mysteries of migration.
In his book Living on the Wind: Across the Hemisphere with Migratory Birds, Scott Weidensaul provides a straightforward answer to one of the questions I had in Belize: Why do birds migrate, anyway?
It isnt cold, it isnt snow, it isnt summer heat or monsoon rains that drive birds to travel, but the pursuit of a full belly, Weidensaul opines.2
But Peterjohn takes a broader view and puts a more scientific spin on the phenomenon: There are different theories why birds migrate. A lot of why has to do with the availability of food supplies [as well as] individual adaptability, and the costs and the benefits of doing so.
The costs are high for long-distance migrants, as are their mortality rates. For those that dont migrate or only go short distances, there may be benefits. For example, their populations may stay steady or improve as long as they can survive the winter.
Many species can tolerate cold weather as long as there is enough food, which includes flying insects, caterpillars, fruits, and nectar. If birds eat only insectsand there are very few who do, says Peterjohnthey must leave the area when the temperatures and insect populations drop, or else they die.
Birds with more adaptable eating habits may stay put when winter comes. For example, woodpeckers, which delight in insects, shift their diets to insect larvae, eggs, or pupae that hide in dead wood, as well as to seeds and other available food.
According to an online fact sheet from the Smithsonian Migratory Bird Center, the ultimate reason for the persistence of migration is that it increases breeding success.
Birds are able to raise more young on average by migrating than they would if they remained in the tropics. The abundant, protein-rich food, longer daylight hours, greater area over which the birds can spread, and, possibly, fewer predators accounts for the potential to raise more young.3
Migratory birds leave their northern breeding grounds in August, when daylight begins to dwindle. (Fear of the dark is not, of course, among the reasons they leave.) Scientists think that certain changes in a birds environment stimulate the production of hormones that, in turn lead, to changes in the birds behavior and physiology, preparing it for migration.4
Part of their strategy may be that they want to pass through temperate areas while there is still an abundance of food, Peterjohn adds. Instinctively knowing that they must leave in August allows them to get the nourishment they need to continue their journeys south.
The required nourishment boils down to a full storehouse of fat. Before long journeys, migrants must gorge themselves to build it up. Some larger birds can even reduce the size of certain organs for a short period and use that internal space to store fat.
To maintain their energy, along the way most make pit stops, which last anywhere from one day to a few weeks.5 Unfortunately, as environmentalists are keenly aware, changes and reductions in natural habitat, and the food that goes with it, can seriously affect their success of finding nourishment.
Habitat changes mean that there are fewer areas for them to refuel, says Peterjohn. Its particularly an issue if they have large areas to cross, such as the Gulf of Mexico. They must have a place to fuel up before they attempt to get across the Gulf to the northern Yucatan without stop.
Not By a Wing and a Prayer
Now for the million-dollar question: How do birds find their way between their breeding and non-breeding sites? The easiest answer is instinct: the abilities needed to migrate seem to be programmed into their genes. Researchers say that songbirds, on their first migratory journey, seem to have a sort of automatic pilot leading them to their ancestral grounds.6 Experience helps them to fine-tune subsequent journeys.
Other birds, like cranes, geese, and swans, migrate in family groups, so its the parents that lead their fledglings to non-breeding grounds and back again for the first time. But even in this case, the first-ever set of birds to fly still had to get the directions from somewhere, and it was probably from their genes.
According to scientific speculation, it appears that an internal map and a compass are hard-wired into their genetic codes. The Sibley Guide to Bird Behavior elaborates: Simply put, orientation is the ability of a bird to use an internal compass to accurately align itself in an appropriate direction . . . Navigation, by comparison, is the ability to find a specific geographical location without using a map.7
In addition to instincts and genes, birds seem to use a variety of environmental cues. Different birds use different cues, explains Peterjohn.
Diurnal migrants (daytime travelers), such as hawks and vultures, rely upon the rising currents of air that enable them to soar and glide. These currents occur only during daylight hours, when the suns rays heat the Earth.
Songbirds and others that migrate at night (nocturnal migrants) may use patterns of stars in addition to the location of the setting sun and the pattern of polarized light it creates.8 Other possible environmental cues include topographic featuressuch as coastlines, rivers and mountain rangesand prevailing wind patterns. Some scientists postulate that even sounds and smells could help them arrive at their destinations.
Its likely that the Earths magnetic fields play a key role in migration. The April 2004 issue of Science included an article on this topic, and heres a quick summary of what the authors found when they studied the interaction of magnetic, stellar, and twilight orientation cues in free-flying songbirds.
We exposed Catharus thrushes to eastward-turned magnetic fields during the twilight period before takeoff and then followed them for up to 1,100 kilometers. Instead of heading north, experimental birds flew westward. On subsequent nights, the same individuals migrated northward again. We suggest that birds orient with a magnetic compass calibrated daily from twilight cues.9
Yet Still a Mystery
Genes, maps, compasses, stars, the Earths magnetic field, and moretheyre all plausible theories of how birds migrate. The more I read, the further I was from finding the answer. However, it gave me some comfort to hear Peterjohn confirm my findings and feelings about a topic that arose in my mind when I saw what I thought was an avian Yankee quite at home in a Belizean jungle.
In a lot of ways, we dont know how they do it, although we are trying to understand, says Peterjohn. One of the most fascinating things is their ability, year after year, to find very specific locations on the ground in both their breeding and non-breeding sites and return yearly. Its mind-boggling. n
1. Phone interview with Bruce Peterjohn, Thursday, February 9, 2006.
2. Scott Weidensaul. Living on the Wind: Across the Hemisphere with Migratory Birds. North Point Press. New York, 1999, 74.
3. Mary Deinlein. Neotropical Bird Basics, Smithsonian National Zoological Park, Migratory Bird Center, fact sheets. (http://nationalzoo.si.edu/ConservationAndScience/MigratoryBirds/Fact_Sheets/default.cfm?fxsht=9).
7. Sibley Guide to Bird Behavior. National Audubon Society, Chanticleer Press. New York, 2001, 64.
8. William W. Cochran, H. Mouritsen, and M. Wikelski. Migrating Songbirds Recalibrate Their Magnetic Compass Daily from Twilight Cues, Science, April 16, 2004: 405-8.
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