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We all know many birds migrate in the spring and fall – 4 billion birds are headed south over the United States this season. (Only 2.6 billion return in the spring; the fall migration is larger because young birds born during the summer fly south with their parents.)
Some birds, such as Canada geese, migrate during the day. They need visual cues to fly in their well-known V formation. If they don’t watch where they go, they crash into other birds or will not stay in the correct aerodynamic position.
Other birds, such as some species of hawks and falcons, use thermal updrafts to convey them on their way, carrying them upward like an elevator before they glide down to find the next updraft, saving valuable energy by coasting over much of their journey. Thermals are stronger during the day, so it makes sense that these birds also migrate during the day.
Many birds, though, especially songbirds, migrate at night, which has many advantages for small birds: There are fewer predators, the moon and stars can be used for navigation, and the birds become less dehydrated in weather that tends to be calmer and cooler. (All that flapping is a lot of work, so birds heat up as they fly. Cool air helps them dissipate heat. In addition, birds do not sweat. Like dogs, they pant to cool down, and in cooler air they breathe slower and therefore lose less moisture as they exhale.)
How do birds know when it is time to migrate? Changes in the length of daylight cause bird metabolisms to change. For a few weeks before they fly, migrating birds eat enormous amounts of food. Some birds, such as hummingbirds, double their body mass. (Captive migratory birds in zoos put on fat when it is time to migrate even though they have nowhere to go and zookeepers feed them less.) According to Audubon Magazine, ruby-throated hummingbirds require the human equivalent of 150,000 calories per day because their heart can beat 1,000 times per minute and their wings can beat still faster – 3,000 times per minute.
Fat accumulation in humans can lead to obesity, diabetes, heart disease or stroke, but migrating birds do not get metabolic diseases. When people run endurance races such as marathons, they burn mostly carbohydrates at first, which is why runners carbo load the night before. Humans can burn fat too, but they are inefficient at it; birds, by comparison, burn fat 10 times faster than people do, and since fat provides much more energy per unit mass than either carbohydrates or protein, they can travel great distances on fat reserves.
How do these seasonally heavier birds even manage to get off the ground? They compensate for their excess weight by decreasing the size of their liver, kidneys, gizzard and digestive tract. And their heart muscle and breast muscles grow stronger.
When blackpoll warblers leave New England in the fall, they fly nonstop for 2,000 miles, reaching South America in only three days. A human would have to run four-minute miles without stopping for three days to exert themselves equivalently. No wonder birds need to fatten up so much. It’s not surprising then that the greater the distance a bird migrates, the more it fattens up.
Studies have shown that migrating white-crowned sparrows sleep up to 85 percent less during migration than other times. These birds are active during the day, foraging for food, then spend most of their nighttime hours flying – and it is believed that sparrows do not sleep while in flight. The little sleep they do get is usually during the first few hours of the night, before they take flight. Somehow, this reduction in sleep during their migratory phase does not cause physical or cognitive problems even though a lack of sleep at other times of year would be a problem just as it is in humans.
Some birds, such as owls, geese and ducks, have an unusual sleep adaptation – and it’s why you might have seen an owl sleeping in a tree with one eye open and the other closed. They have unihemispheric slow wave sleep, which means simply that one side of the brain is awake while the other side is asleep.
Some aquatic mammals are this way too. Half-brain sleep helps ocean mammals surface and breath while they sleep (or half sleep, as it were). This isn’t as restful as full-brain sleep, but it sure beats drowning.
Birds such as puffins spend a great deal of time at sea and sleep there too, bobbing like corks on the surface of the ocean while they snooze. The frigate bird, on the other hand, can fly for 10 straight days over the ocean without stopping. Does it sleep? If so, when? Scientists attached data recorders and discovered that when these birds circle on rising air currents, one side of the brain sleeps while the other side is awake – unihemispheric slow wave sleep again.
The side that is awake is the side facing outward as they circle – presumably where the most dangers await. And even though they manage to eke out a bit of sleep, frigate birds over water sleep only 7.4 percent as much as on land.
Some Swainson’s thrushes migrate 7,000 miles from Alaska to the Andes in South America. Unlike blackpoll warblers that make their journey without stopping, Swainson’s thrushes travel solo and can take months to complete their journey. They compensate for sleep lost at night by entering half-brain sleep for about nine seconds at a time during the day. This doesn’t seem like much sleep, but it’s restorative enough to get them safely to their destination.
The time is now to look for migrating birds passing through our region. If you have questions about their them or their migration, let me know. I’ll do my best to answer.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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