Each year, billions of birds undertake migrations in and out of the United States, with many traveling thousands of miles to reach seasonal destinations.Some,like the Arctic Tern (Sterna paradisaea),accumulate enough miles over their lifetimes to make a round trip to the moon.

But how do these birds navigate such epic journeys?

Birds possess a range of senses that they use to orient themselves, some of which are familiar, while others remain mysterious to humans.

“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, explained.

Sight and smell are fundamental cues for birds finding their way. Birds that have migrated before frequently enough remember familiar landmarks like rivers and mountain ranges. however, birds migrating over water have fewer landmarks and may rely more on their sense of smell. Research suggests that seabirds, for example, use odors to locate food sources up to dozens of miles away.

Magnetoreception and Polarized Light

“We know so little about the details of how this compass might operate,” Hore said.

Beyond sight and smell, birds also possess more specialized senses. One such sense is magnetoreception, the ability to detect magnetic fields. Scientists have long known that birds use the Earth’s magnetic field to orient themselves, but the exact mechanism remains unclear.

One theory suggests that a protein called cryptochrome,found in birds’ eyes,might potentially be responsible. Cryptochrome is sensitive to blue light and reacts to magnetic fields.According to Oxford University chemistry professor, Peter Hore, researchers aren’t sure how cryptochrome is sensitive enough to detect 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 indicates a magnetoreception mechanism in birds’ beaks. Studies have identified receptors in the upper beaks of birds that interact with magnetite, an iron-based mineral. These receptors connect to the brain via nerve pathways, suggesting they could be another way birds gauge the intensity of the magnetic field.

In addition to magnetoreception, birds can detect polarized light, a type of light where the waves oscillate in a specific plane. Sunlight becomes polarized when it scatters through the Earth’s atmosphere in predictable patterns. Birds can sense these patterns using special cells in their retinas, providing facts about the sun’s location, even on overcast days.

Putting the Pieces Together

Birds integrate various compass cues to navigate, with different cues becoming more important at different times. For example, magnetoreception is less reliable during thunderstorms or periods of high solar activity, which can disrupt Earth’s magnetic fields, according to Hore.

Ultimately, a bird’s genetic makeup underpins all of these strategies. Birds inherit the propensity to migrate from their parents, and the distance and direction they fly are primarily based on genetics, according to Liedvogel. Researchers are still investigating the specific genes responsible and how they function.

Both scientists emphasize that understanding these systems is crucial for bird conservation. Relocating or rewilding bird species has become a key focus of wildlife conservation, but outcomes have been mixed. One analysis revealed that in 45% of studies, birds left their new location.

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