You’re probably hearing the birds chirp much more now that it’s spring – the birds are returning from their annual journeys south. But how do they do it?

Migration is a mass movement from one area to another, and many bird species migrate annually. The distances that different bird species migrate vary greatly. Some birds move as little as several meters (down a mountain, for instance) to change elevation while others fly thousands of miles across an ocean.

Birds migrate to chase resources for survival. They follow vegetation for food and locations for nesting, which means that birds usually travel towards warm weather.

However, not all birds migrate. Birds relying on food sources that remain available in the winter do not need to migrate. Insects, a food source for some birds, can often be found on trees in the colder months.

Some birds that used to migrate no longer need to because of changes in winter resources. Cardinals migrated south in the 1920s, but now they brave the Northeastern winter. Some suspect that the rise in popularity of bird feeders caused the change in their migration patterns.

Storks in Portugal used to migrate to Africa for the winter, but landfills now provide them with enough food for the winter.

However, the storks may soon need to begin migrating again. In 2018 Portugal will start switching from landfills to composting facilities.

Migration routes are chosen carefully. Instead of flying the shortest route from their northern home to their southern home, birds stop at points where they know there will be food.

The New York Times wrote of University of Copenhagen bird migration expert Kasper Thorup’s research, “The journey was exquisitely well timed to coincide with high levels of vegetation at each site, he and his colleagues found.”

Migration routes are also chosen based on wind patterns. To choose a direction in which to fly, a species often utilizes a wind pattern to the birds’ advantage.

Long before food becomes scarce, birds know it’s time to prepare for migration. How is this possible? Changes in temperature and daylight hours are thought to inform the birds. There is also evidence that birds’ genes help them know when to leave.

The Cornell Lab of Ornithology writes, “For centuries, people who have kept caged birds have noticed that the migratory species go through a period of restlessness each spring and fall, repeatedly fluttering toward one side of their cage.”

In recent years, improved bird-tracking technology has helped scientists learn more about bird migration. Small GPS trackers can be attached to birds to track their flight, and the Internet lets bird watchers share their sightings with experts.

As technology advances, our understanding of this annual natural feat will grow.