The days are getting longer, and the temperatures are warming up. Spring is almost here, and soon the birds will arrive.
More than 200 bird species migrate northward every spring from their wintering grounds in the southern U.S. and Central and South America.
Here in Mississippi, we are fortunate to be located on the Mississippi flyway, one of four main bird migration routes in the U.S. Our location provides great bird watching opportunities in the spring and fall. So pick up your binoculars and head outside this spring and witness these remarkable animals as they pass through on their annual journey north.
Some travel longer distances than others. For example, the Artic tern travels 7,800 miles one way from its wintering grounds on the southern tip of South America and Antarctica to the high Artic. In contrast, the Clarks' nutcracker lives out West and migrates only a few miles from its high-elevation breeding grounds in the Rocky and Sierra mountains to lower elevations within the same mountain ranges.
Some birds travel a few hours each day and stop to rest, like the blackpoll warbler during its spring migration over the eastern seaboard. But come fall, it will leave the northeast U.S. on an 86-hour direct flight to the northern coast of South America.
Yet some birds, like the Northern mockingbird, don't migrate at all. Simply said, not all birds migrate, and not all birds migrate in the same way. So why do some birds migrate and not others?
Migrating birds take advantage of the seasonal food abundance and favorable climate available in different locations throughout the year. A lot of food is not only good for the individual bird but essential for reproduction and the survival of the young. But millions of birds die each year during migrations from exhaustion, stress, weather, predation and accidents, such as building and wind turbine strikes.
Non-migrating birds have adapted to the stresses of staying put. This includes cold weather, shortages of food and increased competition for resources during different times of the year. But these birds also have early access to breeding grounds and food, and avoid the huge energy sacrifices and dangers of migration.
The bottom line: it is a biological trade-off.
For the birds that do migrate, how do they do it? The answers are fuel, a very efficient engine and an excellent navigation system.
Before migration, birds "pack on the pounds," so to speak. The amount is often only grams of fat, but in some cases it can be as much as 40 to 100 percent of their normal weight. For example, ruby-throated hummingbirds double their normal weight of 3 grams for the 600-mile trek across the Gulf of Mexico.
One unit of fat contains twice as much energy as carbohydrates and proteins when metabolized. Birds store these fat reserves under their skin, in their muscles and other parts of their body, similar to filling up the gas tank and packing a full gas can in the bed of a truck before leaving on a long road trip.
A birds' "engine" includes a highly efficient respiratory system aided by an aerodynamic skeleton and feathers. These three components provide an oxygen-rich system that is light, strong and extremely fuel efficient.
Today's automobiles average 18 to 40 miles per gallon of gasoline. If the blackpoll warbler's "engine" was burning gasoline instead of fat reserves, it would get 720,000 miles to the gallon!
Modern humans navigate with maps, compasses, and the geographic positioning system, or GPS. Birds are excellent navigators without all the gadgets.
Birds use landmarks, such as bodies of water and mountain ranges, as visual cues. During the day, they use the position of the sun. At night, they keep on track by reading the stars.
If this is not impressive enough, they also have the remarkable ability to sense the Earth's magnetic fields. This ability not only provides direction but also their location in their journey.
Birds can also detect barometric pressure, which helps them detect weather patterns while knowing their vertical position in the sky. This is very important, as they can identify favorable wind patterns at different levels in the atmosphere, making it easier to fly long distances.