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The Little Bird that Could

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Hundreds of miles out to sea in a storm, exhausted birds would sometimes land en masse on ships for refuge. Sailors must have wondered at the sight; where did these little birds come from? And what are they doing out here? Did the wind carry them away from the safety of the land? A team of scientists decided to find out.

The blackpoll warbler would fit nicely in the palm of your hand. It weighs about as much as an AA battery, Becky Oskin says in Live Science. Yet this small flyer, not much bigger than a hummingbird, has just revealed its amazing migration story — thanks to a new generation of geolocators small enough to fit onto the bird’s leg. Readers may remember from Illustra’s documentary Flight: The Genius of Birds how geolocators enabled Karsten Egevang’s team to track the pole-to-pole migration of the Arctic tern, a round trip flight of 44,000 miles undertaken each year.

But the blackpoll warbler is much smaller than an Arctic tern. It’s even half the size of the northern wheatear, the previous record holder for lightweight distance migration. Consequently, a smaller geolocator was needed. Weighing only 0.5 gram (two one-hundredths of an ounce), these solar-powered devices captured sun angle and coordinates periodically along the route. Like Egevang, Chris Rimmer at the Vermont Center for Ecostudies and team had to retrieve the loggers the next year. They recaptured 5 out of the 38 birds that had been equipped in Vermont and Nova Scotia the prior fall season.

These striking black-and-white songbirds … perform one of the most extraordinary migrations in the animal kingdom, the researchers said. The warbler’s longest nonstop flight was recorded as being more than 1,700 miles (2,730 km) in three days, the scientists reported today (March 31) in the journal Biology Letters….

"If you account for body scale and size, the blackpoll warbler is the hands-down winner," Rimmer said. "I’m used to being amazed by birds because they do a lot of cool, extraordinary things, but it’s hard to top this one." [Emphasis added.]

The paper says, "This is one of the longest non-stop overwater flights recorded for a songbird and confirms what has long been believed to be one of the most extraordinary migratory feats on the planet."

Lead author Bill DeLuca, at the University of Massachusetts at Amherst, was worried the geolocators would hinder the birds’ migration. "It was pretty thrilling to get the return birds back, because their migratory feat in itself is on the brink of impossibility."

For fuel, the birds fatten up on insects in New England; "They are just sheathed in a layer of fat that makes them look like little butter balls," Rimmer says. How does a butter ball fly? Clearly there’s a tradeoff optimum needed between fuel and weight.

To prepare for the flight, the birds build up their fat stores, explains Canadian team leader Ryan Norris of the University of Guelph. "They eat as much as possible, in some cases doubling their body mass in fat so they can fly without needing food or water. For blackpolls, they don’t have the option of failing or coming up a bit short. It’s a fly-or-die journey that requires so much energy."

But why do they fly over the sea instead of the land? Evolutionary theory was not much help here:

As for why the blackpoll undertakes such a perilous journey while other species follow a longer but safer coastal route, the authors say that because migration is the most perilous part of a songbird’s year, it may make sense to get it over with as quickly as possible. However, this and other questions remain to be studied.

More record-breaking discoveries may be in store. Oskin says that some of the blackpoll warblers spend their summers in Alaska and Canada instead of New England. If they fly east before joining the southern route to Colombia and Venezuela, it would add three thousand more miles to their annual migration.

Elizabeth Pennisi was equally impressed in her write-up in Science, but she says another contender may snatch the gold, if scientists can figure out a way to follow this tinier flyer:

That’s a remarkable feat for a bird the blackpoll’s weight, the researchers say. But an even smaller bird, the ruby-throated hummingbird, which is one-third the warbler’s size, could ultimately beat the warbler’s record, they add. But even the tiniest geolocators are too big for the hummingbird, so researchers have yet to track the hummingbird’s migration over the Gulf of Mexico.

If so, that diminutive hummer will have to wear more gold medals than Michael Phelps, considering all its unique design features illustrated in Flight: ability to hover and fly backward, a unique shoulder joint, a nectar-capturing tongue, its exquisite coloring, and more. Rimmer should be prepared for more "cool, extraordinary things" birds achieve.

But that’s not all. Lest we forget, there are the Monarch butterflies, much smaller and lighter than birds, that travel three thousand miles from Canada to Mexico each year, as detailed in Illustra’s film Metamorphosis. Even though butterflies don’t fly non-stop, that’s — well, we need an adjective beyond "extraordinary." And more recently, British scientists found that another butterfly, the painted lady, flies triple that distance — nine thousand miles — in its annual migration from the UK to Africa.

Think of all the integrated systems that have to be in place to make these migrations possible. The birds or butterflies need elegant flight systems, with all the attendant muscles, nerves, and structural supports required. They require digestive systems to sustain them, circulatory systems, thermal systems, sensory systems, and protection from the elements. Most astonishing are the navigational systems, including sensing the stars or the earth’s magnetic field, to take them unerringly to their destination and back over routes of thousands of miles. To pack all that in a tiny animal is a masterpiece of miniaturization. Oh — and then there’s a reproductive system, and a battery of instincts to operate all the systems.

It seems fair to say these engineering marvels are beyond the reach of unguided Darwinian evolution. The birds don’t have to fly that far. Warblers could stay inland along the coast, and feed on the way, or learn to get by through the winter like other birds do. Butterflies, too, could stay with other species that don’t migrate. Natural selection would take the easy road, get rid of the wings, and let them grovel in the dirt to eke out a living. These migratory abilities are gratuitous and superfluous to the requirements of mere survival. Such over-design, right on the "brink of impossibility," speaks of design so beyond our comprehension, we can only stand in awe.

Image by Andrew C (Blackpoll Warbler (Setophaga striata)) [CC BY 2.0], via Wikimedia Commons.