How do migrating birds know where they're going?

Every year, billions of birds migrate in and out of the United States. And across the world, birds fly thousands of miles to reach their seasonal destinations. Some birds, like the Arctic tern (Sterna paradisaea), even rack up enough miles over their lifetime to fly to the moon and back.

But when birds embark on these epic journeys, how do they know where they're going?

Birds have an arsenal of senses they use to orient themselves — some we're familiar with, and some are still beyond human comprehension.

"We know that birds use a variety of cues to keep their migratory direction," Miriam Liedvogel, director of the Institute of Avian Research in Germany, told Live Science in an email.

Sight and smell are two basic cues that birds use to find their way. If birds have already migrated once, they'll likely remember familiar landmarks, such as rivers and mountain ranges. Birds that migrate over water, on the other hand, have fewer landmarks to guide them. In these circumstances, they might rely more on their sense of smell; one study found that when researchers blocked the nasal passageways of seabirds called Scopoli's shearwaters (Calonectris diomedea), they could still fly over land but became disoriented when flying over water.

Birds also can use the sun and stars as guides. To do this, birds that fly during the day use a "sun compass," which combines birds' view of where the sun is in the sky with their internal perception of what time of day it is based on their circadian rhythm. By integrating these two inputs, birds can determine the direction they're heading, like a living sundial. Research shows that disrupting a bird's circadian rhythm with artificial light prevents them from navigating accurately, showing the importance of the sun compass.

However, most birds actually migrate at night, meaning the position of the sun is of little use to them. In this case, birds rely on the position and rotation of the stars to find their way. They use this star compass by learning the position of the stars around the celestial pole, which is roughly marked by Polaris (the North Star) — the same star humans have used to navigate for millennia.

Related: Why don't all birds fly?

Magnetic fields

But what if the sky is cloudy, and birds can't see the sun, stars or any landmarks? That's when birds' more fantastic senses come into play. Birds can find their way even with no sun or stars, partly thanks to a sense called magnetoreception. This sense allows birds to perceive Earth's magnetic fields, which are generated by the rippling molten metals in our planet's core. This feat may sound like science fiction, but research shows that messing with magnetic fields has a big effect on birds; for example, one study found that altering the magnetic fields around pigeons disrupted their homing abilities.

While it's clear that birds are capable of magnetoreception, exactly how they do it is less certain. Peter Hore, a professor of chemistry at the University of Oxford, said birds must utilize some sort of chemical reaction whose outcome depends on the strength and direction of Earth's magnetic field. There are a couple of candidate theories for how this reaction happens, but Hore's bet is on a molecule called cryptochrome, which is present in birds' retinas.

Researchers have confirmed in the lab that isolated cryptochrome responds to magnetic fields and that this reaction requires blue light, which has also been shown to be necessary for bird magnetoreception. Still, researchers aren't exactly sure how cryptochrome is sensitive enough to pick up on tiny variations in Earth's magnetic field.

"We know so little about the details of how this compass might operate," Hore said. "I mean, we don't even know how many cryptochrome molecules there are in the birds' retinas."

Some research also points to a magnetoreception mechanism inside birds' beaks. Studies have found receptors that interact with magnetite, an iron-based mineral, in the upper portion of birds' beaks. These receptors connect to the brain via important nerve pathways, suggesting they could be another technique birds use to gauge the intensity of the magnetic field.

On top of magnetoreception, birds can gain information about their direction by detecting polarized light — a type of light in which the waves oscillate in a specific, aligned plane. Sunlight becomes polarized in predictable ways when light scatters through Earth's atmosphere. Using special cells in their retinas, birds can sense these patterns, which give them information about where the sun is in the sky, even when it's overcast.

Putting the pieces together

Just as we rely on our vision during the day but may use our hands to guide ourselves around a dimly lit room at night, birds employ different senses at different times.

"Birds likely integrate their compass cues to navigate — and we are pretty sure that different cues are of varying importance during their journey," Liedvogel said. Hore pointed this out as well; magnetoreception, for example, is less useful during thunderstorms or periods of high solar activity, both of which can disrupt Earth's magnetic fields, he said.

Ultimately, all of these strategies are underpinned by birds' genetic drive to migrate. Birds inherit the propensity to migrate from their parents, Liedvogel explained, and the distance and direction in which they fly are primarily based on genetics. Researchers like Liedvogel are still investigating exactly which genes are responsible and how they work.

Both scientists said understanding these systems will be essential to the future of bird conservation. Relocating or rewilding bird species has become a major focus of wildlife conservation efforts, but so far, outcomes have been mixed; one analysis found that in 45% of studies, the birds left their new location.

"Human efforts to relocate those birds have not been very successful," Hore said. "That's partly because they are such good navigators that if you displace them, they simply fly back."

Bird quiz: How much do you know about our feathered friends?