Bird Migration Secrets Revealed
There’s much more to bird migration than meets the eye.
When we were kids we all learned that birds fly south for winter. Even though it was probably one of the first fascinating facts you learned about birds, it’s not entirely true.
Bird migration has captivated and confused scientists and birders for hundreds of years. Early naturalists and philosophers often pondered where birds went in the winter. Some theorized that swallows and other species buried themselves in the mud. Others thought that birds wintered on the moon. At one point, the prevailing theory for hummingbirds was that they migrated on the backs of geese. We know now, of course, that none of these are true. We also know that it’s an oversimplification to simply state that birds fly south for the winter. Let’s explore some of the more interesting migratory fliers.
Migration doesn’t just happen in spring and fall. Winter movements of certain species are called irruptions. Unlike traditional migrations that occur every year, irruptions are more sporadic and driven by local conditions. Available food resources for northern birds, like conifer cone crops for finches or lemmings for raptors, can lead to mass movements of birds in search of food.
Snowy owls are likely the poster species for irruptions. It seems individual snowy owls have fan clubs when they arrive anywhere south of the Canadian border. Less noticeable irruption species include purple finches, pine siskins and redpolls. They might get mistaken for house finches or goldfinches as they nibble thistle seed in backyards across the northern tier of states.
Migration doesn’t have to be a long movement across continents. There are plenty of species that demonstrate altitudinal migration. These birds will move to lower elevations during the harsh winter months. Mountain chickadees can occasionally be seen in towns near the mountains all year, but in the winter, the species moves to lower elevation, visiting feeders in striking numbers. Clark’s nutcracker, Steller’s jay and even the dark-eyed juncos can also exhibit this seasonal movement instead of traditional migration.
While plenty of mountain species move down the mountain during the winter, the dusky grouse of the Rockies moves uphill. These birds survive the winter by nibbling on conifer needles. Then, as the weather warms and food becomes more available, they will disperse back to lower elevations.
These nomadic species don’t move in a classic, seasonal pattern, but instead, wander throughout the year. It’s a highly specialized niche, and there are but a few examples of this behavior. Some populations of red crossbills move far and wide to areas with abundant cone crops. Their unique crossed bills make them dependent on specific food sources and they will move out of areas as food sources deplete. What is especially impressive is that crossbills can adjust the timing of their breeding to take advantage of abundant resources.
While numerous shorebirds and seabirds make epic migration journeys, the Arctic tern arguably wins the gold medal for migration. The Arctic tern follows the summers in both hemispheres. After a short nesting season in the high Arctic, this species heads south—extremely far south. They spend the nonbreeding season near Antarctica. Individuals can cover over 25,000 miles in a single year. That’s about nine trips between New York City and Los Angeles.
Considering its small size, the tiny rufous hummingbird might be the most impressive migrator of all. Rufous hummingbirds have a circular migration route. From their winter ranges in southern Mexico, they work their way up the West Coast. Some will make it as far as southeast Alaska for breeding. Then by July, the males head south. They don’t follow the coast back, though. Instead they move from mountain meadow to mountain meadow down the spine of the Rocky Mountains. Occasionally a rufous hummingbird will drift farther to the east, and a few can be found in the upper Midwest and beyond each fall.
Migration doesn’t flip on and off like a light switch. Many factors play a role in the movements of birds. Competition for nest sites and food availability drive migration patterns throughout the year. The length of days helps trigger migration.
Weather patterns will facilitate day-to-day movements. It’s easier for birds to fly with the wind than against it. Some birds will make epic nonstop journeys, but most take many stops along the way, each providing an opportunity to refuel.
For the irruptive finches and raptors of the north, flying south for winter can mean southern Canada and the northern United States. Neotropical migrants breed in North America and winter in Central and South America. Migration is a complex topic with endless nuances. There aren’t universal rules that apply to all birds. Even populations of the same species will migrate differently in certain instances. Migration is a fascinating topic and one that gets people excited about birds all year long.
Migration in Every Season:
Somewhere on the continent, migration is happening nearly every day.
Waterfowl start to follow the open water northward as soon as they can. Spring migration is in full swing in the south by April, and in the north by late May. In early summer, spring migration is wrapping up for some northbound species while fall migration is kicking in for others. Fall migration sees southbound migrants, including hatch-year birds making the journey for the first time. Seedeaters shift across the regions as winter sets in. Even in winter we continue to search the skies, and our feeders, for winter irruption species, not to mention the resident birds that share our spaces all year long.