Bud’s View: Migration
The Lehigh Valley region’s fall migration is just about finished. A few raptors, like the bald eagle, can still be seen gliding past Hawk Mountain observation points. In most cases, the birds that fly south from the area and the birds that fly south to our region for the winter have settled in.
When I hear the word migration, I immediately think of the north and south movement of birds during spring and fall. But there are other animals that migrate.
I combined several dictionary definitions of the verb migrate and came up with one of my own: The movement of most birds as well as some fish and mammals, from one region or climate zone to another with the change in seasons.
Although it is commonly believed that birds migrate south to avoid the arrival of cold weather, the main reason for seasonal movement in fall is to ensure a continuous and ample supply of food. This is particularly true for birds that rely on insects for the main portion of their diets. Some species of insect-eating birds head south soon after the completion of breeding and the end of their summer molting period. Birds that eat berries, fruits and grains often remain in their breeding habitats until late autumn.
Most salmon species and the American shad move from their adult saltwater habitats in the oceans to freshwater rivers and streams to complete their life cycles. The American eels found in local waterways spawn in the Sargasso Sea, an area located in the middle of the North Atlantic Ocean. Adults die after spawning. The young eels drift with the current toward the Atlantic coastline. Males remain near the coast while the females swim upstream to live the majority of their adult lives in freshwater rivers like the Delaware, the Lehigh and smaller tributaries. Eight years later, the adult eels return to the Sargasso Sea to begin another cycle.
Blue and fin whales migrate twice a year between their polar feeding grounds and their breeding grounds near the equator. Caribou move in great numbers from the northern tundra to spruce and fir forests in the south. Some mammals, like mountain sheep and elk, move to higher altitudes in the summer and back to lower valleys in the winter.
Three species of bats, the red, the hoary and silver-haired, spend time in Pennsylvania during the summer before migrating to warmer winter climates.
Autumn is also the time when the amazing monarch butterflies, observed in our yards and gardens during warm weather, are drawn together forming huge swarms. They escape the cold weather by migrating in mass to central Mexico. After wintering in Mexico, they return north to produce two to three new generations that will follow the same paths and use the same instincts as their monarch ancestors.
Monarch numbers are low because of the loss of wintering habitat and the loss of milkweed plants, the host plant for the monarchs. The trees where the monarchs spend the winter in Mexico are being cut for lumber while the milkweeds are being killed off because of agricultural spraying and new development.
About 80 percent of the more than the 600 species of birds in North America travel from one area to another area during the year. This includes those species that travel up and down mountains with the change in seasons. More than half of bird species are true migrants.
The ruby-throated hummingbird’s story is one of the most amazing migration tales. It is the only hummingbird found in the eastern United States. Hummers depend on flower nectar to support a high metabolism. Before the food supply disappears, these tiny emerald jewels begin migrating to southern Mexico or Central America. Amazingly, they fly nonstop, a flight of more than 500 miles, across the Gulf of Mexico.
North American birds follow four major migration routes. The general route passing through this area begins in the tropics, passes through the West Indies and Florida and north along the Atlantic Coast, then heads west to the central states or on to New England and Canada.
Fall migration along the Atlantic Coast begins in July and usually ends in the Lehigh Valley in late November. The fall migration period lasts more than twice as long as the spring flights back to the breeding grounds.
Birds migrating in large flocks often make plenty of noise, alerting us to their seasonal movement. The honking sounds of snow geese crossing the sky and Canada geese in V-shaped formations are a familiar sight during spring and fall. Birds flying in flocks change positions in the formations. Changing positions allows individuals to share the tiring lead position of breaking through the air and wind.
Many theories try to explain how birds find their destinations. One theory proposes that birds orient themselves using the positions of the sun, moon and stars. Another theory suggests the use of landmarks. Other experts believe birds use the earth’s magnetic field to find their way. The answer is probably a combination of these theories.
No matter what method is used, migration is truly one of the miracles of the natural world. Birds continue to reach their destinations despite inclement weather, predators and other dangers.
That’s the way I see it!
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