Birds that migrate evolved that way mainly because seasonal food sources become too scarce. Insectivores cannot find insects because it becomes too cold for insects to appear. Seed eaters run out of seeds because plants finish their seed-producing cycle for the year or become buried under snow. When lakes and rivers freeze, birds that feed on aquatic plants, fish, snails or other underwater wildlife can no longer seek sustenance.

Shorter days are a related trigger to head south. Birds that feed in daylight have fewer hours to forage at a time of year when they need more calories to keep warm. Deep snow can also hamper a bird’s ability to find food. In addition, leafless trees expose birds to extreme weather conditions and predators.

IT'S A FACT
The Arctic tern migrates farther than any other bird, from its breeding grounds in the Arctic Circle to the Antarctic and back again, a biannual journey of 22,000 miles one-way.

The best option is to fly to a more hospitable spot. Yet migration is fraught with many dangers. Only about 40 percent of migratory birds make it back to their summer breeding grounds each year for these four reasons:

1. Fatigue. Many birds make transcontinental journeys during migration, staying aloft for long periods. Some simply don’t have the stamina to go the distance. As fatigue sets in, others become less alert increasing the chance of getting lost, flying into something or getting eaten by a predator that they didn’t notice.


2. Collisions. Thousands of migrating birds succumb to fatal collisions with man-made objects. Along the coast, lighthouses are common crash sites. Tired birds see the light, fly toward it, become blinded by it and hit these navigational landmarks. Birds also frequently collide with glass buildings, wires and wind turbines.


3. Starvation. After a long time in the air, birds need to quickly eat a lot of food. However, if they touch down in a habitat where they can’t refuel, they won’t recover enough to continue. Bad weather bodes poorly at these critical feeding points in the journey, forcing worn out birds to deal with difficult conditions on top of the need to find food.


4. Weather. Extreme weather knocks birds out of the air or blows them off course, forcing them to fly further, which can lead to exhaustion or the need to land at an unsafe spot. Many birds drown in the Great Lakes, the Gulf of Mexico or in the ocean due to unfavorable weather conditions.

On the bright side, millions of migratory birds make it each year. Migrating is something these birds do as part of their life cycle. For the rest of us, it’s a welcome sign of the new season.