how does a bird know where it's going that high up and in the dark?

Look up: Birds in the night sky

Mon, 06/06/2016 - 3:45am

Step outside on a clear night in late spring, look up at the stars, and listen. You probably won't catch the music of the celestial spheres. But if you listen carefully, you just might hear the chips and chirps of birds. No, these aren't insomniac sparrows; they're spring migrants flying high overhead in the dark.

Many songbirds migrate at night, taking flight after sunset and landing well before dawn. It is believed that flying in the cooler night air helps prevent a bird from literally burning out. The effort of an intense, hours-long flight can significantly raise a bird's body temperature. In addition to being cooler, night air is also calmer, so a bird migrating after dark is less likely to find itself buffeted by the atmospheric turbulence produced by the sun's heat during the day. Daytime predators, such as cats and hawks, can be avoided at night. And during migration it is more advantageous for birds to devote as many daylight hours as they can to finding food and refueling for the next leg of their journey.

Nocturnal migrants generally fly at a few thousand feet in altitude, going higher or lower as weather conditions make necessary. The winner in migration flight height is the Bar-headed Goose, which has regularly been seen flying higher than 29,000 feet — over Mount Everest!

But how does a bird know where it's going that high up and in the dark? While the complexities of avian migration are still being puzzled out, studies have shown that night-migrating birds may use a combination of navigational methods, relying on patterns of polarized light, the earth's magnetic field, their own sense of smell, and/or the stars. (What has become abundantly clear, as well, is that migrating birds can be fatally distracted and disoriented by artificial lighting on skyscrapers, radio towers, oil rigs, etc.)

The prevailing theory is that a bird memorizes patterns and movements of the stars during its first summer. Being a fixed point in the night sky, Polaris, the North Star, is presumably a celestial touchstone for most birds, as it is for most humans learning their star patterns.

A study done in the 1970s showed that Indigo Buntings rely on stars near the North Star to navigate. The circumpolar constellations the buntings seem to depend on — all of which we see year-round here in Maine — include the Little Dipper (which contains the North Star), the Big Dipper, Draco, Cepheus, and Cassiopeia. Young buntings raised in a planetarium with Betelgeuse as the fixed star, however, oriented themselves to the stars around Betelgeuse, instead, so the internal star map is clearly learned, not innate.

While you're out there listening for the calls of thrushes and warblers invisibly winging their way northward, see if you can pick out a couple of stellar birds, as well: Cygnus the Swan and Aquila the Eagle.

Cygnus the Swan, also known as the Northern Cross, is on the rise in late spring, visible above the northeastern horizon. The later you look, the higher it will be; by mid-summer, Cygnus will be almost directly overhead in the Milky Way.

Because this constellation actually forms a cross shape, its identification as a bird makes some sense. The short end of the cross delineates the tail, marked by the alpha star Deneb, which aptly means tail in Arabic. The head star at the end of the long, swan neck is Albireo, which one of my star guides calls the "best double star in the sky." If you've got a telescope, you can judge for yourself.

The star pattern's description as a generic bird, hen, partridge, or even the roc from Sinbad the Sailor's tales predates the Romans' identification of it as a swan. And competing swan stories abound. In one, Cygnus is identified as Cycnus, the friend or brother of Apollo's son Phaëthon. When Phaëthon fell to his death while trying to drive Apollo's sun chariot, Cycnus was so frantic with grief that the gods took pity on him, transformed him into a swan, and set him in the sky (because that always makes one feel better).

Another story recognizes the swan as the form Jupiter took as the lover of Leda, the mother of Castor and Pollux (Gemini) and Helen of Troy. Whichever story you prefer, Cygnus is one big, bright bird.

Aquila the Eagle is very near Cygnus, flying eastward across the Milky Way. Its brightest star Altair forms one of the three points of an asterism—along with Deneb and the star Vega in Lyra the Lyre—known as the Summer Triangle. Altair is the middle star of a line of three, similar to Orion's belt, forming the wings of the eagle.

In the classic myths, Aquila is the eagle that held Zeus/Jupiter's thunderbolts. This eagle also carried out other dastardly errands for the king of the gods, such as kidnapping young Ganymede and pecking out Prometheus's liver. To the Japanese, however, Altair is the herd-boy separated by the Milky Way, the River of Heaven, from his lover Tanabata, our star Vega. The couple is allowed to get together once a year, thanks to a bridge of magpies, on the seventh day of the seventh month, but only if skies are clear.

As star stories go, I prefer the romantic, bird-assisted drama to the liver-pecking regal eagle.

Imagine the stories a newly fledged bird would make up to help itself remember the stars of its first summer nights, to find its way home again next spring.

Kristen Lindquist is an amateur naturalist and published poet who lives in her hometown of Camden.


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