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Fall Migration On The Way
by R. Munguia

As summer comes to an end, one of the most spectacular natural events unfolds in front of our eyes; yet many seem oblivious to this phenomenon. In the late afternoon,  the flocks of northern crows seem to cross our cities like rivers of black dots.  The days now are shorter and many birds prepare for the long journey. Inside their brains a set of instructions in the form of chemicals is sent to promote the accumulation of fat. By now the birds are done with breeding and perhaps are wearing new feathers they recently molted, a sign of the approaching adventure. Even before the first signs of winter appears in the landscape, millions of birds have embarked on what for us humans may be the trip of a lifetime.

The Blackpoll warbler flies from as far as Alaska to its wintering grounds in Southern Brazil.

Welcome to fall migration, one of the most strenuous events in nature. Forced by the lack of food on the northern latitudes, many species of birds have no choice but to head south.  Some will fly a few hundred miles, but for most this epic journey takes them more than 5,000 miles away from their summer home. Meet the Golden Plover, a shorebird that spends most of the spring and summer in the  Arctic circle regions, but flies its way to southern Brazil for the winter.  Its flight path takes him over the Atlantic on the way south, and then over Central America during spring migration. But this bird is a good sized bird when compared to warblers and hummingbirds; yet these tiny birds take on the same challenge. Some may not go as far as Brazil, but still have to cross over the Atlantic to reach South America.  All this is possible thanks to their fat reserves. A typical ruby-throated hummingbird starts migration with a weight of four and a half grams, of which more than two are just fat (Lasieswki 1963).  Fat is nothing more than stored energy for the long flight that will take about 26 hours non-stop over the Gulf of Mexico before it reaches the Yucatan peninsula more than 650 miles away.  The Blackpoll warbler, a slower flyer than the hummingbird, will endure an 85 hour flight over the Caribbean Sea before it reaches the north shores of South America.  Once the birds arrive, they need to replenish their fat content quickly. Now they spend several days just feasting on the available food. Some will continue further south, but at least the flight over land is not as challenging as over the ocean.  Many birds succumb to the stress of flying such long distances, many dying at sea, especially if they encounter bad weather or become disoriented.  It is not strange to hear reports from sailors who were visited by an exhausted warbler, hundreds of miles offshore.  But the record holder so far in the longest migration accomplished by a bird belongs to the Arctic tern, which flies about 24,000 miles every year in its migration from its feeding areas back to its breeding grounds in the Arctic.

The Swallowtail kite flies more than 4,000 miles from Florida to South America where they spend the winter.

Although migratory routes seem to be fairly consistent year after year, there are many factors that may cause an occasional shift in the route or the time at which the migration starts for a particular species. Climate change will most likely produce changes in migratory patterns.  While many birds migrate during the day, birds such as the warblers, thrushes, vireos, orioles and the majority of sparrows tend to fly at night. What causes them to switch their habits during migration? Several reason have been proposed to explain this behavior, especially important when these birds are strictly diurnal.  Perhaps the clue lies in the fact that many birds use stars and constellations as fixed cues for navigating to their final destination. Based on this assumption, most birds will fly south on clear nights whenever possible. Yet many seem to do fine even in overcast skies.  As of today, there is no evidence to  support that birds benefit from the moon for navigation at night.  Research suggest that many species are capable of perceiving the earth's magnetic fields and use these for reference in their night flights. An added advantage of flying at night is that most raptors are also flying south, but these predators of the sky only migrate during the day.  While migrating, they feed on other migrants flying the same routes.  Perhaps the predator pressure has caused many birds to fly at night as a way to avoid the raptors; besides, it will be cooler to fly at night, giving the bird more endurance. For diurnal migrants, the sun plays a large role in navigation, and so do the contours of the land which are used as a visual reference.  Birds are known for adjusting their flight routes to compensate for shifts in the wind direction, and even change flight altitude to benefit from a tail wind.  Interestingly enough, diurnal migrants tend to fly with a headwind, while night migrants prefer a tailwind.

Not all birds migrate. Depending on their survival needs, some species will remain within their summer range. Birds in tropical climates may not have the need to migrate long distances, but rather in small movements to areas with available food.  Many birds that live in Canada and the northern United States will fly as far south as Florida, but will rarely continue further south. The American White Pelican, a mainly freshwater bird, resides in the Great Lakes and North Dakota during summer and flies as far south as the Florida Bay for the winter.  Once in Florida, they move from lake to lake in search of food or even move to the coast line where brackish water abounds.  Then by late April, they start flying back. At this point, their reproductive hormones tell them is time to head back; some will developed a horny plate on their bills, a sign that they're sexually matured. This is also the cause for the reverse migration of many other birds, something known as spring migration. The weather up north has cleared and new food sources are sprouting.  Most of those birds that once left the confines of North America are now flying back to their breeding grounds. Once they arrive, they'll build a nest if required and will raise a new generation, that in time will fly down for  their first fall migration.





Facts About Bird Migration

Changes in the length of daylight trigger a chemical response in migratory birds to start building up fat.

Not all birds are migratory. Resident birds are those that stay in the same area through all seasons.

Weather as well as human or nature altered ecosystems can cause problems to migrating birds. A lack of food resources or resting areas along a flyway can have negative effect on migratory birds.

There are four 4 major North American flyways that have been named the Atlantic, the Mississippi, the Central and the Pacific Flyways.
Birds may use a combination of cues including sun and star positioning, visual or magnetic cues, and even wind currents.
North American Flyways
Best Places to Watch Bird Migration
Bosque del Apache, New Mexico is one of the best places to watch the spectacular migration of Snow Geese and Sandhill Cranes.
Cape May, New Jersey is well know as a stopover for many migratory birds including warblers and raptors.
Delaware Bay and the barrier islands in VIrginia are a stopover for shorebirds and most notably the Red Knots.
The Florida Keys offers a good view of raptors flying south.
Fort DeSoto Park, Florida is well know for spring migration birds.

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