The mysteries of migration

May 26, 2010 • 10:04 am

The more we learn about bird migration, the more bizarre it gets.  If you’re at all interested in birds, or in animal behavior, get your tuchus over to The New York Times and read Carl Zimmer’s nice piece on how recent technological advances, like the invention of tiny geolocators, have produced surprising new results about migration.  A few snippets:

Just as he had suspected, the bar-tailed godwits headed out over the open ocean and flew south through the Pacific. They did not stop at islands along the way. Instead, they traveled up to 7,100 miles in nine days — the longest nonstop flight ever recorded. “I was speechless,” Mr. Gill said. . .

Consider what might be the ultimate test of human endurance in sports, the Tour de France: Every day, bicyclists pedal up and down mountains for hours. In the process, they raise their metabolism to about five times their resting rate.  The bar-tailed godwit, by contrast, elevates its metabolic rate between 8 and 10 times.  And instead of ending each day with a big dinner and a good night’s rest, the birds fly through the night, slowly starving themselves as they travel 40 miles an hour. . .

In fact, ruby-throated hummingbirds returning north in the spring will set out from the Yucatán Peninsula in the evening and arrive in the southern United States the next afternoon. .

Mr. Gill and his colleagues have recorded similar odysseys from other wading birds, using satellite transmitters. They found that bristle-thighed curlews fly as far as 6,000 miles without a stop, traveling from Alaska to the Marshall Islands. They have also recorded whimbrels flying 5,000 miles nonstop from Alaska to Central America. . .

By the time the birds are ready to leave, their bodies are 55 percent fat. In humans, anything more than 30 percent is considered obese. But as soon as the birds are done eating, their livers and intestines become dead weight. They then essentially “eat” their organs, which shrink 25 percent. The birds use the proteins to build up their muscles even more.

And the weird thing is that we have no idea how the birds find their way.

Fig. 1.  A geolocator.  Weighing less than 1.5 grams, it can track a bird’s position by monitoring ambient light levels.

Fig. 2.  A juvenile shrike wearing a geolocator.  From Ontario Field Ornithologists.

14 thoughts on “The mysteries of migration

  1. Ruby-throated Hummingbirds don’t count, since they, you know, ride on the backs of Canada Geese.

  2. Carl Zimmer must be a hoot at dinner parties. “Hey everybody, put a lid on the financial news, Zimmer is going to tell us about a 30 foot long worms burrowing out of our intestines! More fettuccine alfredo, anyone?”

    (Reading Parasite Rex, are we? Yes, why yes I am.)

  3. While it isn’t in the same league as migrations by individual birds, every year at about this time (saw the first one day) turtles migrate from a pond/wetlands area near us across a black-topped road to a farmer’s field. They must have been doing this for ages, since before the road was built, and can’t seem to stop. The road has gotten busier and busier in the 30 years we’ve lived here and it seems inevitable that the turtle population will be whittled down. I know there are lots of examples like these.

  4. Beyond amazing, but one thing that wasn’t mentioned was how do they obtain water in flight? Sure, flying thru rain must provide some, but is that sufficient?

    The reason I mention this is that I know for sure that pigeons can survive at least a week in temperate summer without access to water. I wound up running the experiment when some pigeons found their way into the Romanesque tower of Andrew Carnegie’s first library that I’m involved with, when a window in it blew open. They stayed there and wouldn’t leave – there was no idea of just leaving the window open because I’d never get there when all were gone. I figured I’d just come back the next day and then they’d be happy to leave when I opened the window but they just stayed in the rafters and looked down at me. I returned each day and repeated the process but they just kept staring at me. Finally, after a week of that (heartless athiest that I am) I put one of those Hav-A-Heart groundhog traps up there with a dish of water. They were inside the trap the next day. There was absolutely no access to any water up there, and it didn’t rain that week either. I have yet to meet a bird person that seems aware of this capability, but if anyone can shed any light on it I’d appreciate knowing more about this.

    I find it hard to imagine that metabolically-generated water is sufficient to satisfy the need, but that’s the only thing anyone comes up with.

    1. Also birds don’t urinate like mammals do, I believe they don’t sweat, and they have efficient flow through lungs. All of these likely minimize water loss.

      Perhaps more importantly, metabolizing fat doesn’t use up water as glycogen does, doesn’t it?

      All hail Wikipedia, it had an example: “if the human body relied on carbohydrates to store energy, then a person would need to carry 67.5 lb (31 kg) of hydrated glycogen to have the energy equivalent to 10 lb (5 kg) of fat.”

  5. I can’t help loving the terminology –

    – bar-tailed godwits
    – bristle-thighed curlews.

    You can’t make it up – but someone did!

  6. So terns go twice the circumference of Earth every year? No wonder they accumulate so much frequent moonflier miles!

    we have no idea how the birds find their way

    What a funny thing to say after referring to an article that presents precisely such an idea (sensing the angle of Earth magnetic field). I’m sure that meant to say something else, but not exactly what.

    1. Oh, I get it now, it’s one of those figures of speech.

      But new technology should eventually permit testing those ideas, surely?

  7. Makes you wonder what doof of a bird got such a nutty idea.

    “Hey everyone! Let’s risk life and limb to fly 10,000 miles to some God-forsaken place every single year! And back!”
    (All) “YEAH!”

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