Migration is a fascinating phenomenon. Not all birds do migrate, as permanent residents will not fly south in the winter and north in the spring; they stay within their territory all year. Other species might move shorter distances staying closer to their original territories, and still more species will fly much longer distances.
We are going to talk about the birds that do migrate, why, where and how in the United States.
Photo Credit: ©Danny Hancock
Some of the reasons that birds migrate south are: food availability, weather/ temperature, day length, and geography.
The reason that birds migrate north is not as clearly understand by scientists, but some theories suggest a genetic predisposition and location fidelity. New studies show that migration programming in birds is incredibly complicated involving neurological, psychological and behavioral traits.
In addition, birds who nested successfully in a particular breeding site will show a strong urge to return to the exact location.
Before birds take off on their long journeys, they prepare to migrate by fattening up on berries, insects, seeds and fruits in the late summer and tropical offerings of food in the spring to travel north.
After the breeding season some birds will lose their worn feathers (molting) and grow clean fresh feathers for the long flight south. During their migration a songbird can lose up to half of their body weight. Some species like swallows eat on the wing during migration.
How do birds navigate between their breeding ground and their overwintering stay?
Birds use the earth’s magnetic field, geographical landforms, waterways, the stars and the sun, seasonal weather patterns and even city lights learned through evolution, to help lead them to their destinations.
However, in many cases, birds do not survive the migration due to manmade circumstances. These causes are in addition to natural threats such as predators. Birds that navigate at night have been determined to use the North Star as a reference. Birds that migrate in the day use the sun as a reference, knowing that the sun moves from east to west.
As mentioned, birds have amazing navigational skills. Scientists have recently determined that certain bird species can see the earth’s magnetic field. The birds studied possess a special protein in the eye that is sensitive to blue light which allows them to visually detect the field.
Where do the birds migrate? Birds in the United States that are in long distance migration are going to southern Mexico and into Central and South America, depending on the species.
There are four flyways that birds in the US use to migrate: Pacific Flyway, Central Flyway, Mississippi Flyway, and Atlantic Flyway. Two of them are in the East and two are in the West.
Birds who use two of the four flyways (Pacific and Central) move at a much slower pace using river corridors for cover and food. They fly over land their whole journey. This is not as difficult for them because they don’t have to go long distances without stopping like the birds in the east that fly over the Gulf. Some of those Eastern migrants jog over to the west to avoid the Gulf to get to their destinations.
The mountain valleys that birds navigate over can provide lift (thermals) allowing the birds to use less energy during their flight. As a rule, the migrating songbirds leave around sunset, flying between 2000-5000 feet. Larger birds like sandhill cranes and geese fly as high as 20,000 feet. Some long distant migrating songbirds will fly nonstop over the Gulf to the next landfall, where they will forage and rest.
Other birds such as sandhill cranes fly over land and some come as far away as Siberia.
The mortality rate of birds in general is high. Birds often perish due to the extreme demand that is needed to make an eleven-hour flight during migration. Unfortunately, due to high-rise buildings in large cities with windows that stay lit throughout the night, tens of thousands of migrating birds fall victim to this peril each spring and fall.
In addition to manmade obstacles, there is also the threat of predators such as owls and hawks. When birds land after long flights over water they can fall victim to outdoor cats. The findings from research on the phenomenon of migration is adding more and more knowledge as to how and why birds migrate and adds to just how complex and fascinating the bird world is.
About the author:
Wezil Walraven is a field ornithologist and a professional bird guide with over 40 years of experience. Based in Sandia, New Mexico, Wezil owns and operates Wezil Walraven Bird Tours and guides throughout the Southwest.
Find him on Facebook.