Migration

Migration: (n)
Definition:
1. The act or an instance of migrating.
2. A group migrating together.
or:
1. The seasonal movement of a complete population of animals from one area to another. Migration is usually a response to changes in temperature, food supply, or the amount of daylight, and is often undertaken for the purpose of breeding. Mammals, insects, fish, and birds all migrate. The precise mechanism of navigation during migration is not fully understood, although for birds it is believed that sharp eyesight, sensibility to the Earth's magnetic field, and the positions of the Sun and other stars may play a role.
or:
the periodic passage of groups of animals (especially birds or fishes) from one region to another for feeding or breeding
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It's essentially a movement of something from one place to another. With animals, it's usually an annual occurrence.
With birds, it happens twice a year. Once going north to their breeding grounds and once going south to their wintering grounds.

So why to birds migrate?
This answer is from the Texas parks and wildlife website:
"There are a number of explanations for migration. (1) Birds migrate to areas where food is more abundant, (2) there is less competition for nesting space, (3) the climate is milder, or (4) the daylight hours are longer. These enhance the chances of survival of a bird and its brood. Most birds require a rich, abundant supply of food at frequent intervals because of their high metabolic rate. Adequate food is not available throughout the year in most regions. North American birds must endure the hazards of winter or migrate to more friendly climates. In winter they migrate to the warmer, southern regions of the United States, Caribbean, Mexico, Central America and South America where food is abundant. In the spring, these birds fly north to habitats where spring and summer provide more food production and less competition for food and nesting sites than in their winter habitat. Summertime at northern latitudes also means more daylight hours to seek food for themselves and their nestlings." (http://www.tpwd.state.tx.us/huntwild/wild/birding/migration/faq/#a2)

So there's your answer there. Most birds migrate not because it gets cold out, but because of lack of food. Most of the landbirds that migrate are insect eaters. Warblers, Thrushes, Flycatchers, ect. In winter, the insect supply drops and most of these birds can't subsist on seeds alone. There are those of course that can subsist on mostly seeds and many of those stay year round. You know many of them from your backyard feeders. Chickadees, Nuthatches, some Sparrows, Finches, ect.

So that leads us to the next question:
How many species of birds in the USA migrate?
The answer is approx 2/3 of the entire number of species found in the USA. In numbers, that's somewhere around 500 species. This includes Shorebirds, Seabirds, Raptors and Passerines.

Which bird has the longest migration route?
This question is a tricky one since study on migration is only very recent and actually tracking migrating birds with radio tags has only come into play in the last 10 years or so. Just to give you an idea, here's an article that was published in the New York Times on Jun 20, 1915: http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9900E6D7123FE233A25753C2A9609C946496D6CF&oref=slogin

Back then, the Arctic Tern (Sterna Paradisaea [The Tern of Paradise]) held the record for the longest migration. 11,000 miles from the Arctic to the Antarctic.
However, 11,000 miles is the distance if you go due south from one pole to the other. The Tern's actual path certainly does not trace a route due south and may even meander much more than we think. It's possible that the actual distance that the tern travels may as much as 25,000 miles or more. (http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Arctic_Tern.html)
One Arctic Tern, ringed (banded) as a chick on the Farne Islands off the British east coast, reached Melbourne, Australia in just three months from fledging, a sea journey of over 22,000 km (14,000 miles).
Currently, our tracking devices are too big and heavy to fit on these small birds so until we can make them smaller and lighter, we will not know that actual distance that this bird travels.

Currently, the recordholder for the longest migration route is the Sooty Shearwater with a migration distance of 40,000 miles! (http://news.nationalgeographic.com/news/2006/08/060808-bird-migration.html)
The Shearwaters winter off New Zealand and fly a roundabout path northwards to their breeding grounds in the North Pacific.
They have a wingspan of 43" and can travel as much as 620 miles in a single day.

The current recordholder for the longest single non-stop flight of a migrating bird is the Bar-tailed Godwit. Last year, ornithologists tracked a female of this species flying 7,145 miles from it's breeding grounds in AK all the way to the wintering grounds in New Zealand. (http://news.nationalgeographic.com/news/2007/09/070913-longest-flight.html)
My friend Andrea over at Earthbirds just posted something about this:
http://earthbird.blogspot.com/2008/10/hats-off-to-e7.html?showComment=1224775080000#c8193636909369642248

Even though there are wayward flyers like the ones I described, most migrants follow an established route from one location to another. There are five major flyways in this part of the world. The Atlantic Flyway, the Mississippi Flyway, the Central Flyway, the Pacific Flyway, and the Pacific oceanic route. Here in WI, I live on the Mississippi flyway. It is a major route for Passerines, Ducks and hawks especially.

Which birds are the highest flyers during migration?
Your first thought to this question might be Hawks and Eagles. They're always flying up high. But not really. Most birds only fly as high as they need to to complete their journey. Most bird migration is in the range of 150 m (500 ft) to 600 m (2000 ft). (http://en.wikipedia.org/wiki/Bird_migration)
However, Bar-headed Geese hold the record for the highest flyers. They have been observed flying several hundred feet above the highest peak on Mt Everest. That's above 30,000ft!

So how do birds navigate their way to where they're going?
There are several different theories on this. Some birds are believed to use the angle of the sun's rays as a compass and as a sign telling them when to go. Other birds are believed to use the earth's magnetic field the same way we do. This second theory is the more popular one.

How does temperature and weather affect migration?
Temperature is another factor is determining when to leave. As the seasons go on, the average temp steadily drops. Woodcocks and snipe use temperature in determining when to leave.
Here's something I found on how weather affects migration:
"Weather is one of the chief external influences on migration. Cool air masses moving south in the fall can trigger migratory flight. Cool air brings high pressure, low or falling temperatures and winds moving in the direction of flight and clear skies. If the cool air meets warmer air, clouds, precipitation and fog may result. Fog, especially, causes birds to descend to the ground and cease migration. Sudden changes in the weather can be disastrous for birds. In the spring a warm, moist mass of air (low pressure with higher or rising temperatures) moving north over the Gulf of Mexico can start a wave of migrating birds to move northward from the American Tropics or southern United States. A southward moving cold front meeting such a warm air mass can result in heavy rains and high winds. This can stop migration immediately or within 24 hours. These spring "fallouts" or "groundings" of migrants may occur when the migrating birds literally fall into sheltered areas seeking food and refuge. This can be disastrous if the migrants are forced down into the ocean drowning thousands of birds. Resumption of southerly winds and rising temperatures starts migration northward again." (http://www.tpwd.state.tx.us/huntwild/wild/birding/migration/faq/#a2)

How did migration originate?
The answer again comes from the TX parks and wildlife website:
"The roots of the migratory habits of modern birds are believed to date back millions of years, and were tempered by environmental changes caused by the Ice Ages of the Quaternary period over the last 2,500,000 years. Migration, as is known among modern birds, probably developed gradually by stages. As the environment changed, some animals changed their habitat slightly, hardly leaving their home region. The movements of others were more erratic, moving toward more favorable places. These first stages of migration were stabilized by natural selection. As winters grew more severe, much of a given bird population probably perished rather than attempting to flee any unfavorable conditions. A fraction of this population probably sought more favorable conditions elsewhere. Natural selection favored the 'migrants' and migratory tendencies were retained."
(http://www.tpwd.state.tx.us/huntwild/wild/birding/migration/faq/#a21)


Migration here in WI is slowly winding down. Most of the Passerines have already gone south and most of the ducks are quickly following. The winter residents are starting to arrive now. Snow Buntings, Lapland Longspurs, Finches and others are filling in the gaps that the migrants left. The winter finch forecast for this year looks good. Ron Pittaway (The finch expert from Ontario) has put out his forecast for the upcoming winter season: http://ebird.org/content/ebird/news/ron-pittaways-winter-finch-forecast-2008-2009
On Mon, Hawk Ridge Bird observatory counted the following non-raptors:
30 Snow Geese, 3 Cackling Geese, 2 Black-backed Woodpeckers, 2164 American Crows, 36 Common Ravens, 99 Snow Buntings, 28 Rusty Blackbirds, 20 Purple Finches, 744 White-winged Crossbills (record high count), 2 Common Redpolls, 566 Pine Siskins

Interestingly, they had a record high count of White-winged Crossbills. These finches depend on the soft cones of Black and White Spruce and Hemlock. The seed bank for these trees is down this year and cone production is low compared to past years. We may see a moderate irruption of this species into Northern and Central WI and surrounding states.

So that brings us to the final migration question:
Do birds that are usually non-migrants also move?
The answer is yes! Chickadees, Cardinals, Nuthatches, Finches, Owls; they will all move to where food is more plentiful. Also, there is general seasonal movement within the ranges of these birds. I suspect that the chickadees that you see in winter, are not the same birds you see in the summer. The same thing with Goldfinches. There is definitely at least some seasonal movement with this species. Especially at the southern extent of their range. In the summer, there are no Goldfinches in TX, GA, MS or some of the other southern states. In the winter, you can find them there. However, in places in the northern extent of their range, they cease to exist.

So in the end, most of the reason that birds move from one location to another is plentiful food or lack of it.
So think about it. If you're hungry, where do you go? to the store where there's more food. Birds don't have a grocery store so they have to move elsewhere where food is more plentiful.

I hope you learned something. I certainly did.

Happy Birding!

P.S.

books used in this post were: 

Field guide to birds of North America: Kaufman, Kenn

Published by: Houghton Mifflin

Dictionary of birds of the United States: Holloway, Dr. Joel Ellis

Published by: Timber Press, Copyright 2003, ISBN: o-88192-600-0

Websites used:
http://www.tpwd.state.tx.us/huntwild/wild/birding/migration/faq/#a2

http://query.nytimes.com/mem/archive-free/pdf?_r=1&res=9900E6D7123FE233A25753C2A9609C946496D6CF&oref=slogin

http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Arctic_Tern.html

http://news.nationalgeographic.com/news/2006/08/060808-bird-migration.html

http://news.nationalgeographic.com/news/2007/09/070913-longest-flight.html)

http://earthbird.blogspot.com/2008/10/hats-off-to-e7.html?showComment=1224775080000#c8193636909369642248

http://en.wikipedia.org/wiki/Bird_migration

http://ebird.org/content/ebird/news/ron-pittaways-winter-finch-forecast-2008-2009