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  #1  
Old 05-02-2001, 11:30 AM
tcline76 tcline76 is offline
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Every year bird fly south for the winter in the fall to avoid the cold and barren conditions of winter and instead enjoy the fruitful and warm south. Why do they fly back north in the spring? You'd think they'd just stay south and save themselves a hell of a lot of commuting time.

Tim C.
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  #2  
Old 05-02-2001, 11:33 AM
tiny ham tiny ham is offline
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avoiding tourists?
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  #3  
Old 05-02-2001, 11:39 AM
Akatsukami Akatsukami is offline
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Because there simply aren't enough resources (food, nesting spaces, etc.) in the tropics to go around. Failure to fly back to the temperate zones would doom a considerable number of birds to death, and an even greater number of genetic lineages to extincton.

Even if we supposed that migratory bird numbers were reduced to a point where they could co-exist with native tropical species, and their instincts to migrate when receiving the proper signals were removed, the temperature and arctic zones would, in their proper seasons, present a huge source of unexploited resources. The first bird (indeed, the first few thousand) to move north (or south) would live the life of Riley, far away from the relentless competition of the tropics. And so the evoluntionary cycle wouls start again...
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Old 05-02-2001, 12:30 PM
screech-owl screech-owl is offline
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First, not all birds migrate. Some birds stay in an area year-round. Take a look at some of the maps in a Peterson Guide - the Eastern Screech-owl (yours truly) and the Great Horned Owl are listed as 'year-round' residents

Second, most migrate to follow food sources, not to get out of the cold: some travel thousands of miles (Peregrine Falcons), other are short-trip (Gyrfalcons are rarely found below mid-Canada and listed as 'accidental' south of the US border). Why do they nmove? Because their food sources (smaller birds) are migrating!

Third, many species have a half-go/half-stay population: most of the juveniles will migrate early, whereas adults will stay as long as the food holds out.

Some excerpts from the Atlas of Bird Migration:

Quote:
If it (the bird) stays in the northern hemisphere, the weather will be less severe; if it crosses the equator, it can enjoy the southern summer. [vice versa for southern species.] This results in a long migration to an area where the summers may be too hot to breed, but where the winter conditions are ideal.

Many other, less immediately obvious, factors have, over geological time, caused birds to alter their movements and influenced the evolution of new species....

...continental drift affects distribution and migration patterns....It is no coincidence that the most complex migration systems are those that have developed between Eurasia and Africa, while those between North and South America, and between Asia and India, are simpler.

[There is a very long explantation of the succession of ice ages affecting migration - to sum up:]...[Changes in climate]...may affect both its summer and winter quarters, but for a long-distance migrant, the effects are more noticible in the northern, summer breeding area. Birds can cope with gradual change because individuals, from year to year, have to modify their journeys only on the edge of the range....
Birds that migrate to a tropical area are competing for food, water and shelter with the species that are normally there year-round. This is usually a good incentive to leave, unless food is provided, as the case with non-migratory Canada goose: farmers and hunters would leave grain in the fields, attracting more Canada geese. With no reason to leave (plentiful food) the geese stay; many have bred and become a nuisance in the area where they settled.


As a side note to this, many birds may not make it to their normal migratory areas: much habitat in the normal stop-over areas have been altered or developed, leaving fewer stopover rest and refueling areas, meaning fewer birds make it to the wintering grounds and back. Various pesticides are still used legally and illegally in many parts of the world: some are fat-soluble, and as the bird uses stored fat for energy, the toxins are released into the body and BAM! dead bird falling out of the sky. I've just finished reading a couple of monographs on tower kills (radio and cellular tower strikes), and the stats ain't pretty. Illegal hunting of protected species for fun and/or profit still exists (we have several eagles permanently crippled by people who didn't, or didn't care to, fully identify their target first). And before you say I blame humans for everything, yes, predators (cats, other carnivores, and larger predatory birds), inexperience, bad weather, exhaustion, and disease do take out a good number of birds yearly.

Sorry for the rant: I'm at work and have been in the library shelving 'pesticides' for the last half-hour. I needed a break.
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Old 05-02-2001, 01:01 PM
Podkayne Podkayne is offline
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Quote:
Originally posted by screech-owl
As a side note to this, many birds may not make it to their normal migratory areas: much habitat in the normal stop-over areas have been altered or developed, leaving fewer stopover rest and refueling areas, meaning fewer birds make it to the wintering grounds and back. Various pesticides are still used legally and illegally in many parts of the world: some are fat-soluble, and as the bird uses stored fat for energy, the toxins are released into the body and BAM! dead bird falling out of the sky. I've just finished reading a couple of monographs on tower kills (radio and cellular tower strikes), and the stats ain't pretty. Illegal hunting of protected species for fun and/or profit still exists (we have several eagles permanently crippled by people who didn't, or didn't care to, fully identify their target first).
Don't forget wind turbines. You thought wind power was totally green? Sure, it is, until some endangered purple-toed warbling dabchick hits the rotor blades. Whoosh, dead bird. Depressing, isn't it?
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  #6  
Old 05-02-2001, 01:33 PM
screech-owl screech-owl is offline
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Quote:
Originally posted by Podkayne
...Whoosh, dead bird. Depressing, isn't it?
Sigh. Yup. My day was starting to look better: now I have something else to think about. Though I do like the "purple-toed warbling dabchick" (Royadactyl expressivii).

[sarcasm]
At least Orlando isn't advanced enough to have many wind turbines (none, as far as I know). Heck, we voted down light rail because some residents would have to idle their Mercedes at an RR crossing for an extra minute in the morning and the afternoon.
[/sarcasm]

Got any reports or stats on those kills? I can add it to our library.
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  #7  
Old 05-02-2001, 03:48 PM
CrankyAsAnOldMan CrankyAsAnOldMan is offline
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Quote:
Originally posted by screech-owl


Second, most migrate to follow food sources, not to get out of the cold: some travel thousands of miles (Peregrine Falcons), other are short-trip (Gyrfalcons are rarely found below mid-Canada and listed as 'accidental' south of the US border). Why do they nmove? Because their food sources (smaller birds) are migrating!

Yeppers. I think that this is one of the biggest misconceptions about birds: that they migrate because of the cold. It's not cold; it's food. Snow on the ground covers up food sources, and/or seasonal foliage changes mean some foods are no longer available. When the Canada geese come through, we have more or less of them depending on how much snow the areas north of us have. While migrating they mostly eat grain out of the fields, and as the snow hits, they move further south. I suppose that given their druthers, many of them would stay up in the arctic circle all year if the lack of food didn't require moving. They don't mind the cold at all.

What I don't understand is why so many of them migrate all the way down to Texas when places like, say, Oklahoma don't have the "snow-covering-food" problem. Seems like they could stop there before flying back.
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  #8  
Old 05-02-2001, 03:53 PM
Rayne Man Rayne Man is offline
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Daylight Hours

Another reason they fly north in the summer is the longer daylight hours.If they stay in the tropics they have 12 hours of daylight.When they come back here to the UK they have nearly 18 hours in the summer.This gives them more time to find food to feed their young.Most migratory birds are the ones that feed on insects (swallows etc.) and they need all the time they can get to catch this food.The baby birds are just one big eating machine.
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  #9  
Old 05-02-2001, 04:03 PM
Ross Ross is offline
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Speaking as an idiot who thinks he understands complex issues, and knowing next to nothing about any animal life, I reckon it's for the same reason we return home after a holiday. Our visiting Hawaii/Mallorca/Scarborough is a carefully-balanced arrangement depending on the good of both communities and strictly-observed rules of arrival and departure. If we hung around and started taking jobs/food, our welcome would cool off, as indeed it sadly does for immigrants and even refugees the world over. Same thing true in animal kingdom, maybe no?

The fact that we don't consider such factors more than fleetingly on holiday is about as relevant as is the fact that birds don't understand how their systems evolved either. For the birds, and for us... it's just how it's always been.

Except for smartarses like we who frequent the Dope. We are the loud man on the plane who keeps explaining it to everyone. We are the annoying bird at the back of the flock who won't stop explaining aerodynamics. We... are the future.
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