One of the interesting things about spending the summer going back to the same walking trails to listen for birds (I use the Merlin app), is noticing when different birds disappear from the songscape. Yellow warblers and Eastern kingbirds, which had been my vocal companions throughout the summer, disappeared at the end of August, replaced by yellow-rumped warblers and eventually, dark-eyed juncos as migration changes summer range for one species into winter range for another.
More than 4,000 birds, or 40% of all species, are migratory. Driven primarily by changing food resources, in the spring, birds move northward to take advantage of emerging food sources, reversing their path in the fall as food becomes more plentiful in the southern hemisphere. This quest for food drives billions of birds to fly from one end of the globe to the other. Last night, like every night for the past month or more, hundreds of millions of birds flew across North America, leaving their summer breeding grounds and heading south. Because migrating birds travel primarily at night they pass largely unnoticed, except by those who are watching for them or when they are killed by smashing into glass buildings as happened in Chicago this week.
Migration, whether it’s the journey of ruby-throated hummingbirds traversing the Gulf of Mexico or the 71,000 km trip that Arctic terns take between Greenland and Antarctica, requires birds to use an incredible amount of internal resources, so much so that many birds have acquired specific adaptations to aid their journeys.. Some species double their body weight in preparation, even going so far as to absorb unnecessary organs, such as parts of their digestive tracts, into their bodies to allow for more energy stores. Sleep is also sacrificed, species such as Swainson’s Thrushes, fly through the night, napping on the wing for up to nine seconds at a time. Not all adaptations require physiological changes. Some researchers are speculating that some birds may be using human transport, and specifically, boats, to aid their migrations.
Why do birds migrate such long distances? Why not simply pick a point in the middle and save the energy? The answer, in part, appears to be related to the longer days that occur in the north during the summer months. Individuals who migrate north are able to raise larger broods of chicks compared to those that stay put in the tropics.
One of the big mysteries of migration is how birds are able to navigate on their lengthy journeys, especially considering how many birds migrate at night. Flying at night allows birds to take advantage of calmer air and lowers the risks of predators and overheating. But how do birds know where to go in the dark? Various theories suggest that birds may navigate using their sense of smell, by sensing magnetic fields of the earth, or by using the stars for navigation. Definite answers are still elusive. There is evidence that different birds use different methods or may even use these methods in combination.
Even though we may only get a glimpse of their migration, long-distance migrants are making astounding journeys. The calliope hummingbird, which weighs less than a marshmallow, travels 8,000 km from British Columbia to Mexico. Even more incredible, a bar-tailed godwit (a type of shorebird, similar to a sandpiper) flew more than 13,000 km from Alaska to Tasmania in a single trip.
Not all birds fly direct. Stopover sites provide important habitat where birds can refuel, building up their energy for the next leg of their trip. These sites are not chosen at random, instead, birds will return to the same site every year, making the identification and protection of these areas important for bird conservation.
Whether they fly or hitch a ride, bird migration is one of the most awe-inspiring events in the natural world. No species flies higher than the bar-headed goose, which flies across the Himalayas or further than the Arctic tern, whose zigzag route makes it cover a distance greater than the circumference of the Earth in single migration. Models of persistence, mere months from now, they will turn around and do it all over again.