The urge to feed and breed drives birds across oceans, mountains, and the equator. Some migrations originated after the last ice age in response to the retreat of ice sheets in areas where birds could expand their ranges to exploit food resources and decreased competition during the breeding season.
Then, they flew south again for the winter when temperatures dropped and the flowers and insects on which they fed were gone.
Some birds fly for several days in a row without stopping, sleeping, or eating! One of the tiniest migrants, the Ruby-throated Hummingbird, flies 900 kilometers nonstop across the Gulf of Mexico with the benefit of aerodynamic flocks like most species.
The Bar-tailed Godwit was named the “endurance champion of the animal kingdom” after completing a nine-day nonstop 11,000 kilometer flight across the Pacific Ocean. This sort of journey requires consuming energy at around eight times their resting metabolic rate.
By comparison, a professional cyclist may function at around five times BMR… but only for a few hours.
Even birds get hormonal! Before migration, birds undergo hormonal changes that literally change their physiology. Changes in hormone levels cause them to fatten up and build energy reserves to sustain flight. Then, just prior to departure, the digestive system shrinks because the birds may not eat again for several days.
Conversely, the flight muscles and heart increase in size and capacity. The birds also molt into their stunning breeding plumage. The bright, beautiful colors help them attract a mate and the fresh new feathers are strong for sustained flight.
Migratory birds have internal clocks cued by changes in day length around the spring and fall equinoxes. The instinct is so great that even migratory birds held in captivity can get fat, molt, and exhibit Zugunruhe, or migratory restlessness, during migration season.
They flutter their wings and try to fly in the direction in which they would normally fly. But when the captive birds are exposed to unnatural patterns of daylight, they do not get Zugunruhe, demonstrating their innate response to environmental cues.
On the other hand, juvenile birds do not molt into breeding plumage or migrate even though they are experiencing the same environmental cues because they are pre-pubescent and have not experienced the changes in hormone levels yet.
How do the birds know in which direction to fly? How can they correct for winds and storms that blow them off course? Birds have sun, star, and magnetic compasses. The sun and stars make predictable rotations across the sky, providing direction both day and night.
When the sky is cloudy and birds cannot see the sun or stars, they rely on their sense for the Earth’s magnetic field. They are sensitive to the angle at which lines of magnetic force meet the Earth’s surface, which changes at different latitudes.
Some birds that were captured, shipped to areas outside of their flyway, and released have still been able to reorient themselves and fly to their original destination!
Once shorebird chicks have hatched and are feeding on their own, the parents fly back to the southern hemisphere for the winter. When the chicks have developed their wings and feathers, they too make the journey south.
The amazing thing is that they have never made this trip before. And they do not have an experienced bird to show them the way. Somehow, they just know where to go.
They fly up to 10,000 km over land or water they have never seen before. The European Cuckoo migrates across the world without ever having contact with its parents, providing evidence that birds can navigate by instinct.
Despite their amazing adaptations, the birds still face many threats on their journey. Bad weather systems, predators, and habitat destruction prevent as many as half of migrating birds surviving to their destination. When they arrive at their stopover points, they are starving and exhausted from days of flight and fasting and depend on those rest areas persisting from year to year.
Because the birds are international species, conservation efforts require international cooperation. Things you can do to protect migrants include keeping your cat indoors, turning off lights in office buildings at night, buying shade-grown coffee, and keeping bird baths free of mosquito larvae since West Nile Virus is an increasing threat to birds in North America.
Want to see birds at rest and in flight? Mary has identified the 10 Birding Hotspots for bird watchers.