Photo: Prothonotary Warbler, Kyle Dunbar/Audubon Photography Awards
Migrants Are Coming
Tom Blackburn
The next week signals the start of a massive surge of migration into Northern Virginia. Each spring, 3.5 billion birds and 350 species cross the southern border of the United States on their way to nesting grounds in the northern states and Canada. Each fall, 4.7 billion birds, their numbers swelled from successful breeding, move south to their wintering grounds in Central and South America. Many of the birds have already crossed the border and are making their way northward in stages. The first Purple Martins, flying from Brazil, are already here according to reports from eBird. And our winter migrants – Dark-eyed Juncos, White-throated Sparrows and others – are heading north, to return in October and November.
The distances birds travel during migration are little short of unbelievable. Ruby-throated Hummingbirds, weighing 1/8 of an ounce, fly 500 miles nonstop across the Gulf of Mexico. Bar-headed Geese have been observed flying at an altitude of 23,500 feet in the Himalayas, where humans have difficulty walking without oxygen. A Bar-tailed Godwit was recorded flying 8,400 miles without stopping on its 11-day journey from Alaska to Tasmania, Australia. And Arctic Terns, the champion long-distance migrants, fly 18,000 miles round-trip every year.
Long-distance migrants’ physical adaptations to accomplish their journeys are also little short of amazing. The Bar-tailed Godwits are perhaps the champions at modifying their bodies for migration. They double their body weight from 1 pound to 2 pounds, largely in fat to be used in their flight, while shrinking their gizzard, kidneys, liver and gut to make room for the extra fat supplies. Many birds also shrink their reproductive organs to 1% of their breeding season size when they are not needed.
How the birds accomplish their migration is astonishing and still being figured out by scientists. A bird can use landmarks to guide its way, but many first-year birds migrate alone, without any memory of the correct route to take. Birds can use the sun as a guide, but many birds migrate at night. Using the stars also helps, but to do that a bird has to take into consideration the changing position of the stars relative to the earth as the earth rotates on its axis. Some birds can sense the earth’s magnetic field and can use minute changes in the field to help keep them on the right course. That sense can be critical when winds sweep the birds far off their intended path.
The hazards of migration are substantial. Many of them such as predators and severe weather are unavoidable, but we humans can reduce bird deaths in one critical area: collisions with buildings and disorientation from lights. Birds flying at night can be confused and disoriented by bright lights on tall buildings. Last fall, more than 1,000 birds were killed in a single night when they flew into the side of the McCormick Place Convention Center in Chicago. More than 1 billion birds are killed each year in the United States as a result of flying into buildings. Many of these deaths can be avoided by reducing building lighting during the spring and fall migration seasons. Even more deaths can be avoided by following the Five Principles for Outdoor Lighting promoted by DarkSky International. April 2-8 is International Dark Sky Week, timed to coincide with the spring bird migration season.
How can you experience the wonders of bird migration? Birdcast uses weather radar, meteorological changes, and years of data compilation to predict periods of high migration and show the numbers of migrating birds in real time. On the night of October 6, 2023, Birdcast recorded a record-high 1.2 billion birds in flight in the United States. If you would like to experience bird migration first-hand, instead of via computer, a prime spot to look for warblers is Monticello Park in Alexandria. You can learn about the park, as well as read predictions of what species of birds are most likely to be at the park each day during migration season, at MPNature, a website created by local birders Bill Young and Ashley Bradford. If you are willing to travel, Magee Marsh in northwest Ohio is renowned for the numbers of warblers who stop there to rest and refuel before crossing Lake Erie. You can learn about it and a 10-day birding festival on Cornell’s All About birds website. Of course, you can also go to any local park, bring your binoculars and keep your eyes peeled. The next seven weeks are the best time of year to go birding since birds are dressed in their best, brightest breeding plumage.