How Do Birds Know When to Fly South and Where to Go?

Most people at the end of the day know when it's time to go to bed. This is because humans have an internal clock that alerts us that we need rest. Birds have a similar internal cue, says Dr. Jason D. Weckstein, associate curator of ornithology at Drexel University. For birds to know when it's time to migrate, one of the big cues is the change in amount of daylight, he explains. "In the Northern Hemisphere as fall is coming on you get a shortening in daylight. That cues bird's desire to move south," he says, noting that this is called "migratory restlessness."

Indeed, the change in day length, also known as photoperiod, is something that birds are extremely sensitive to, says Dr. Andrew Farnsworth, senior research associate at the Cornell Lab of Ornithology in an email. As the days get shorter or longer, "many changes begin to occur in birds' biochemistry and physiology," he says. "Presumably the connection between day length and such changes and migration resides deep in the evolutionary history of many animals." So, they might be inspired to migrate, but don't necessarily know why because it's so engrained in their psyche.

Bird species whose migratory patterns are largely influenced by photoperiod are known as "obligate migrants," Farnsworth says. "[They] start their movements perhaps long before conditions locally with weather and food might drive them to move." This "hard-wired" behavior results in pretty predictable migratory patterns, including distance traveled, timing and returns.

The other type of migratory birds are known as "facultative migrants," which migrate only when they need to for survival's sake. The migratory practices of such birds are "governed by more dynamic processes of their environment," Farnsworth points out. Such birds make haste for the south when they sense that the weather conditions are becoming too difficult, or when the food supply starts to dwindle. Some facultative migrants hit the road one year if things are looking bleak, but not the next if everything's tolerable, so it's really dependent on their personal assessment of environmental factors.

"The metrics are all over the ballpark!" Farnsworth says. "Some birds may move hundreds of meters up or down slopes in altitudinal migrations. Other species migrate thousands of kilometers one way, even between 10,000 to 20,000 kilometers [6,214-12,427 miles] roundtrip for some species."

Some of this, he says, is due to atmospheric conditions, which can allow such birds to travel huge distances efficiently enough. In other cases, the birds are following their evolutionary history and returning to places where their species originated. Still others decide to go to areas that are the most conducive to helping them produce lots and lots of offspring. Lastly, dominant species are more likely to keep going back to "maintain their territories," whereas others might give up and decide that the trek is not worth it.

One example of birds with an impressive migratory range, according to Weckstein are Arctic terns, which migrate, "pole to pole, Arctic to Antarctic." This is a round-trip journey of 18,641 miles (30,000 kilometers) and is in fact the longest distance any animal on Earth migrates! Another species that goes all out are Blackpoll Warblers, which fly up to 72 hours without stopping over a great expanse of ocean, all the way from the Atlantic coast north of Vermont to the South American mainland. For perspective, each one of these itty-bitty birds weighs only 12 grams, or less than a half-ounce. That's a serious trip for such tiny creatures, yet they do it over and over again without fail.

Many human beings have a hard enough time finding a place across town without the help of Google Maps. So how do birds manage to find an obscure location, potentially thousands of miles away with their little bird brains? Scientists are still working to definitively answer this question, but they have some theories.

First, many, but not all, birds migrate to the same general location year after year. Some even use the same places for pit stops along the way! They are able to do this because they have an internal map and compass of sorts that helps them figure it out. These come in the form of many different cues, Weckstein says, which helps them navigate. "It's like an airplane that has more than one GPS unit on it," he explains. "If one goes bad you have the backup to save you."

For example, he says that many birds migrate at night, so they use the stars and actual constellations to find their way. When the sun is visible, they also use its position in the sky to orient themselves.

Both of these methods are of course common ways that humans have navigated land and sea for most of time. However some birds can also detect the magnetic field generated by Earth's molten core, he says. Somehow, using this information, they are then able to figure out where they are and where they need to go. Some scientists theorize that birds may have pigments in their eyes, which "let them visualize the magnetic field," Weckstein says.

There's also some belief that birds' bills might also have structures that help them in the migration process. Then, there's the fact that birds who migrate during the day actually learn landmarks, which help guide them on their path again and again. All in all, it's pretty obvious that the whole "fly south for the winter" concept is much more complicated than most of us ever realized.