A semipalmated sandpiper drinks at sunrise in Bar Harbor on Aug. 23, 2023. Shutterstock
The fall migration is sneaking up on us. Already most of our breeding swallows have departed the Pine Tree State for warmer wintering areas. A diversity of shorebirds can be seen on intertidal sand flats and moist lake and pond edges. They are just passing through.
Most of our migratory perching birds move through North America in fits and starts. They may migrate 50-100 miles south during one night and then linger for a few days until they have refueled, and the weather is favorable.
But for those who will winter in Central America or South America, a big hurdle waits for most of them: the Gulf of Mexico. This trans-oceanic route cuts the distance of getting into the tropics compared to flying westward overland and then south into Mexico and beyond. A successful flight over the Gulf is a non-stop flight. Birds that lose their strength and land on the water will perish.
These flights are marvels of endurance but pale in comparison to some of our other species. Semipalmated sandpipers wend their way from their arctic breeding grounds to get to the Bay of Fundy to fatten up for an arduous trek. Some of these sandpipers will make it into New England, but relatively few go further south, Instead, most make a non-stop flight over the ocean all the way to the productive mud flats at the mouth of the Amazon River in South America. This flight requires four days of continuous flight.
Blackpoll warblers are the northern-most breeding warblers. They migrate in greatest numbers during the fall along the eastern seaboard. They can be abundant in New England but oddly enough are quite uncommon further south in the United States in the fall. We now know that blackpolls follow the same strategy of the semipalmated sandpiper in undertaking one huge flight to get to their South American wintering grounds.
The ability of birds to undertake long migration is astounding. They have several metabolic challenges to meet. First, the resting metabolism of a bird is higher than a mammal (also warm-blooded) of similar size. Smaller birds like hummingbirds, warblers and sandpipers have higher metabolisms than larger birds. When a bird flies, its metabolism increases as much as eight times its resting rate. The best you can do with maximum exertion is to double your resting rate. So, a long-distance migrant needs a lot of fuel and a way to carry sufficient fuel for its flight.
Fat is the fuel of choice for migrating birds. Fat has two big advantages over carbohydrates or proteins as a fuel source. First, the combustion of a gram of fat yields more energy than either a gram of protein or carbohydrate. Second, the breakdown of fat produces more water as a byproduct. This metabolic water sustains a migrating bird that has no way to drink in flight to replace lost water.
The fat for migration is carried just below the skin on the underside of the body. But packing on the right amount of fat is tricky. A bird needs to have enough fat stored to fuel its migratory leg, but carrying an excess means that flight efficiency will be low because of the greater weight carried. This calculus is not critical for birds flying over land; they can simply land and feed in the morning.
Trans-oceanic migrants are at more risk. We know that semipalmated sandpipers double their weight from 20 grams to 40 grams before embarking on their long migration. Birds captured in Suriname as they end their four-day flight are just about at their lean body weight.
The gut and gonads of migrating birds regress before migration. Every bit of weight reduction helps.
Migrating birds use the wind to their advantage. In the fall, a high-pressure system pushing against a low-pressure system is advantageous for southward migration. The counterclockwise winds of the low-pressure system and the clockwise winds of the high-pressure system both blow south where the two systems come together. That is go time for migrating birds.
Semipalmated sandpipers and blackpolls leave North America by flying southeastward out to sea. They will ultimately reach the trade winds, dependably strong wings that will help push them to South America.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at [email protected]
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Publish date : 2024-08-24 13:00:00
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