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The Mystery of Bird Migration

Birds that breed in the northern hemisphere, especially those in regions with definite seasonal differences tend to migrate, even travelling down into the southern hemisphere. The Arctic Tern makes the longest journey, migrating from the North Pole in the fall down to the South Pole and then back again in the spring. Birds who breed in the southern hemisphere also migrate. However very few, except perhaps sea birds, migrate from the southern to the northern hemisphere. The warm temperatures close to the Equator are the wintering home of many birds although many will choose a spot that is not as far but still warmer and has more food than where they left.

What triggers birds to start off on their journey? How can they travel such long distances? How do they know where to go? These questions have mystified people from the earliest times. You can even find references to it in Aristotle and Homer's writings as well as references to it in the Bible. And although we have learned much, there is so much more that we are learning each migration season.

Fall migration allows birds to move to a different location so that they will continue to be able to find food. In the spring they return to the places where they will breed and raise their young. It may be the slant of the sun's rays, hormonal changes, the change of the weather or other factors that contribute to the birds' urge to migrate to their other home.

Hungry birds, however, do not set off on migrations. If their food supply diminishes in a region, they may move to find a better source of breakfast, lunch, dinner and snacks. But it is not the immediate lack of food that triggers the seasonal migration. A bird needs a reserve of fat within its body to be able to travel the long distances, especially for those birds that attempt non-stop or very long flights. These fat deposits are partly due to hormonal changes that store the food necessary for the bird to survive migration. Those that migrate in shorter trips to the final destination are less dependant on adequate and abundant food prior to the trip and need not store the large fat reserves.

Many small birds may even double their body weight before migration while larger birds, such as the Canada Goose will gain proportionally less than this. (If a Canada Goose did somehow gain the same percentage as a warbler, the poor goose would not be able to take off and fly!) These small birds store enough fat that many can fly non-stop for unbelievable lengths of time - sometimes up to 90 hours! In the best of conditions during migration, their body may only use less than a half gram of fat per hour.

Last September, I travelled down to Rockport, Texas to witness the migration of the Ruby-throated Hummingbirds. These tiny birds, less than 6 inches long, travel to a staging place, such as Rockport on the Gulf of Mexico. They feed on the flowers and from the feeders, "tanking up" so to speak. Then, when the conditions are right, they set off over the Gulf of Mexico, flying 500 miles non-stop to the Yucatan peninsula. They may be flying 30 or more hours without touching down.

When are the conditions right for migration? The winds must be blowing in the direction that the birds want to go. Birds will usually wait until the most favorable weather conditions then set off on their journey. Ornithologists watch the weather to help predict when large numbers of birds will be passing through their area. A strong wind in the right direction will speed the birds on their way. If there are strong headwinds, the birds' speed will be greatly reduced, therefore it will need more fat reserves to travel the same distance. Many birds that encounter storms and strong headwinds perish into the seas. If these weary birds are over land, they will drop down and land, find food and rest before continuing.

The route that some birds instinctively take may seem strange until you look at the winds. For instance, many birds on the east coast of the U.S., head out to the Atlantic during a fall cold front. The northwest winds take them on a southeastern course over Bermuda and beyond. Then they meet the northeast trade winds and make it to South America. This unbelievable journey will take them over 1,800 miles of water and will last over 80 hours. But it is actually easier for them than the land route along the coast, down through Central America, and then onto their destination in South America. The winds are the key factor here.

Flapping of the wings uses quite a bit of their stored energy. Therefore, many birds, especially the larger ones, will soar and glide as much as possible, using the winds and thermals. However, the journey will take longer than if the migration was made by more flapping flight. Spectacular kettles of hawks are seen at various places where thousands of hawks climb to the greatest height possible then glide and ride the thermals, soaring without flapping for great distances.

How high do birds fly during migration? Pilots have reported seeing birds as high as 26,000 feet! A bird's lungs are different than human's so the colder air at these high altitudes actually helps the birds. However, most birds fly less than 3,000 feet in the air.

Many people are unaware of the thousands of birds that may pass in the skies near them. Part of the reason may be that many birds travel at night. Many birders watch the full moons and count the birds cross in front of it. Birds may elude their predators by this night travel. Those birds that do not fly non-stop, will usually land in the very early morning hours, find a safe place to rest and will be able to find food during the daylight hours. Night migrators include vireos, various waterfowl, sparrows, cuckoos, warblers, thrushes, and flycatchers. Those who prefer a daytime trip include hawks, eagles, pelicans, storks, swifts, swallows, shrikes and some finches.

How do birds know which way to go? How do they stay on course? Various studies have been conducted that suggest that birds use the sun or stars as guides. Birds may also use the physical characteristics of the land below, following rivers, shorelines, foothills and valleys as guides. There is also the "homing" instinct that has been studied. There have been studies where birds have been taken in closed boxes to remote locations and released. The birds found their way back to their colonies. Check here for a wonderful article about this. Homing pigeons are thought to use smell to help navigate.

Birds do get lost sometimes, thrown off course by huge storms, fierce winds, or other weather conditions. Young birds, perhaps those who fledged late in the season and unable to migrate with the older birds, may become confused and stray.

One of my very favorite places on the WWW is the Operation Migration site. Here you can learn about a group of scientists teach birds to fly with an ultralite, leading them during migration. (The movie "Fly Away Home" was based on these activities of ornithologists.) So far geese, Sandhill Cranes and Whooping Cranes have all been "led" by these ultralites.

There is much still to learn and know about migration. Banding of birds, observations, and radio tracking by use of small transmitters on the birds all gather information and add to the knowledge.

Another recent way of gathering information is the use of radar. Birds will show up on radar, even migrating Monarch butterflies will appear. How many birds, their migration route, speed, and more can all be gathered from radar. A group of birders in Texas have been discussing the use of Nexrad radar which is available on the WWW. Check here to get to the Texbird archives. Click on the messages having to do with radar and Nexrad to see what these fine birders are saying about radar and migration.

In my opinion the ultimate migration site on the WWW is Journey North. Information on migrations of orioles, robins, loons, eagles and hummingbirds as well as Monarch butterflies, whales, and manatees are all found here.

What can you do during migration? Be sure to keep your feeders full for hungry and weary travelling birds. Plant bushes and trees for cover and food as well as flowers for seed to eat. With the development of land and destruction of birds' natural habitat, there are fewer and fewer places for these migratory birds on their way north and south.

For more information on migration, be sure to check my links to other migration articles on the web.

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