Semipalmated Sandpiper (Photos by Don Reimer)
Semipalmated Sandpiper (Photos by Don Reimer)
The months of August and September initiate the fall migration period for many types of birds. For some species migration happens in graduated, short-hop steps, while others accomplish their impressive feats in a matter of a few days or weeks. What factors allow birds to move so readily across regions or even continents? Certainly their lightweight skeletal structure, strongly developed flight muscles and aerodynamic wing design are critical elements. But more is involved for birds.

I’m thinking about their highly developed circulatory and respiratory systems. By relative size, birds have larger hearts than all vertebrates, sometimes proportionately double in size to similarly sized mammals. With each pump of the bird’s highly efficient heart muscle, the ventricles are almost fully emptied, rocketing oxygenated blood to all body systems.

For purposes of flight efficiency, birds’ lungs are smaller and more compact than mammals’. They are much denser, however, and weigh as much as some larger animals’. Unlike humans, birds do not have a simple in-out breathing cycle. Instead, air passes in a somewhat continuous flow through a complex series of internal air sacs routed back to the lungs. Consequently, a nearly complete exchange of air volume results with each subsequent breath.

How do these physiological factors foster travel over vast distances? In general, powered migrants have a wider availability of altitude ranges than soaring birds that must rely on warm air thermals for lift. Much of bird migration occurs at altitudes of 2,000 to 3,000 feet or higher. Endurance flight shorebirds tend to favor the higher strata to gain advantage of reduced air resistance in the slightly thinner upper atmosphere.

Let’s consider two diverse species that transit Maine en route to the tropics each fall. Small migrant flocks of Semipalmated Sandpipers are now gathered in the mid-coastal marshes and mudflats. Mostly juvenile birds making their first passage, these 6-inch sandpipers will spend a couple of weeks fattening and building energy reserves for the non-stop 2,000-mile trans-oceanic journey to the northern coast of South America — depending on ambient weather and atmospheric conditions, a sustained flight of 40 to 80 hours.

Coastal flocks will often begin their trek at peak high tide since they can no longer forage the flooded mudflats. After several aerial spirals to discern the compass bearing, the purposeful flocks proceed over the southeast ocean. Prevailing westerly tailwinds aid their swift passage until they pass over the Equator. Then northeasterly breezes exert control to shepherd the flocks onto South American terrain.

Although shorebird migration is astounding enough, another small species, the Blackpoll Warbler, makes an equally arduous intercontinental jaunt. In September, migrating Blackpoll Warblers accumulate between Nova Scotia and New Jersey awaiting the arrival of powerful fall cold fronts that facilitate an express ride to the West Indies or South America. With a typical warbler flight speed of around 20 miles per hour, the migration of this length would require about 100 hours — a physical impossibility. With additional 30 mph northwest tailwinds provided by the cold front, however, that same warbler would average a 50 mph ground speed and reach its destination within 40 hours.

According to a nature account by David Allen Sibley, “When measured in terms of the energy spent per distance traveled, avian flight is highly economical. It has been estimated, for example, that a songbird flying a distance of 1 kilometer expends less than 1 percent of the energy that a mouse would use to run the same distance.” For an average human, running a kilometer at 8 mph will burn about 76 calories. We humans will clearly need to take a plane to South America.