It catches us off guard every year: spring migration is over and the brightly colored birds that birders spend all winter anticipating and studying have begun nesting and rearing the next generation of migrants. Birders may lament the inexorable march of time after the latest stragglers from the south finish arriving; however, good things come to those who wait, as the excitement of fall migration is not months or weeks away. It is happening right now.
Unlike spring migration, a contracted window spanning several exciting weeks to a few short months, “fall” migration is a protracted experience, starting as early as mid-June and lasting until the early days of January. Such a long migration season provides birders plenty of opportunities to witness the spectacle and joy of birds on the move. While peak fall migration occurs from mid-August to mid-October, the middle of summer is remarkable for its own migrations.
Here are a few examples and the fascinating mechanisms that drive them.
In the West, hummingbirds are among the earliest migrants to head south. Male hummingbirds do not help raise young, enabling individuals of some species such as the Rufous Hummingbird to leave their breeding territories in the Pacific Northwest by the middle of June. Females typically follow a week after their young fledge, and juvenile birds migrate after the adults. The first stop is to nearby higher elevations in search of flowers, which are more available there during the heat of summer as a rich source of nectar. By early July, many of these Rufous Hummingbirds arrive in parts of Arizona on their way to wintering grounds in Mexico. They are soon joined by other species, providing peak hummingbird diversity like nowhere else in the United States with the possibility of seeing 10 to 14 species in early August.
Arctic and Boreal-breeding Shorebirds
By the end of June, shorebirds, such as the Least Sandpiper or Short-billed Dowitcher, begin to arrive south of their breeding ranges located in the Arctic and boreal regions. Often, these early migrants are adult birds that failed to nest. Because the nesting season is so short at the higher latitudes where these shorebirds mainly breed, if the first attempt at nesting fails, they cut their losses and begin their long migration south. By mid-July, many adults that managed to successfully raise young begin their journey back to their wintering grounds. By late July, the first few juvenile shorebirds begin to arrive, distinguished from the adults by their fresh plumage. Finally, by August, shorebird migration is in full swing across the United States, providing a dazzling array of identification challenges for birders across the country.
Heermann’s Gulls only nest on select arid islands in the Gulf of California, including Isla Rosa, where up to 95 percent of the population breeds. After breeding, these gulls migrate over the Baja Peninsula to the Pacific coast before heading north in a “reverse” migration, reaching as far north as southern British Columbia by July and August. Heermann’s Gulls time their post-breeding migration out of Mexico with that of the Brown Pelican, and their post-breeding dispersals, to rob them of their fish in a great example of kleptoparasitism. As summer turns to fall and winter, these species migrate south again, back to southern California and Mexico in search of warmer weather.
After breeding, many species of herons and egrets tend to move north and inland from their southern or coastal breeding haunts. These dispersals provide many birders opportunities to spot these magnificent birds away from their breeding areas in mid to late summer along some of our inland waterways. Sometimes, these post-breeding wanderings are true vagrancies, such as during the summer of 2018, when birders discovered Roseate Spoonbills and Wood Storks from Minnesota to Maine. Quite often, many of these errant individuals are juvenile birds, although not always.
Monsoon Molt Migration
Along with breeding and migration, molt is one of several energetically costly processes that birds experience during their annual cycle. To balance these events, birds often breed, migrate, and molt at different times of the year, employing different strategies for each. One unique strategy, called molt migration, sees birds migrating specifically from one area, often their breeding range, to another in search of better resources to fuel their molt ahead of completing their migration. In the arid regions of the West, where resources are limited in the sweltering-grip of late summer, populations of Warbling Vireo, Bullock’s Oriole, Lark Bunting, and other songbird species migrate to the monsoon-rich region of the desert Southwest and adjacent northwestern Mexico. By early August, seasonally predictable rains in the region create a "second spring," where vegetation becomes lush once again, temperatures cool, and food replenishes. These resources help fuel the molt for these species during a critical portion of their annual cycle.
Endless Summer Shearwaters
Every northern summer, millions of seabirds that breed in the Southern Hemisphere are on the move. Chasing an endless summer as trans-equatorial migrants, these masters of migration cross into northern oceans in search of food and warmer weather. Perhaps no other species are as famous for this as the Pacific Ocean populations of Sooty and Short-tailed Shearwaters. These salty seabirds breed around Australia and New Zealand generally arriving at their breeding colonies in October, laying eggs in November, and fledging chicks by late April. Departing the oceans of these areas in the southern fall, they arrive in Japan, Alaska, and California by May and June. By mid-summer in the Northern Hemisphere, millions of Sooty Shearwaters are found just offshore in one of the greatest migration spectacles in North America. By early in the northern fall, the Sooty Shearwaters head south again, crossing the equator once more on their way to another breeding season “down under.”
Wherever you live, be sure to look for these, and many other amazing summer migrations. Much of our current knowledge on bird migration is the result of careful observation by birders, such as you, during all months of the year, including summer.
Additionally, as tracking technologies continue to improve, researchers such as those that have contributed their data to Audubon’s Migratory Bird Initiative will help tell the stories for more of these marvelous migrations. With advanced technology allowing for an increased precision in migration knowledge in both time and space, Audubon and partners can help protect migratory birds and the places they need throughout their annual cycle, now and into the future.