Not So Secret Life of Sea Birds : Migration and Population Trends

In its lifetime, the shorebird known as a bar-tailed godwit flies a distance that is equivalent to  traveling from the earth to the moon and almost back again. Scott Weidensaul, author of A World On The Wing: The Global Odyssey Of Migratory Birds, has offered proof of Greek philosopher Aristotle’s notion that “the more you know, the more you know you don’t know.” As an example, ponder this factoid from Weidensaul’s book: the intestines and digestive organs of some birds shrink prior to their seasonal long distance migrations. At the same time their heart, lungs and leg muscles double in size. I am sure I didn’t know this. I was equally unprepared for many other revelations that Weidensaul shares.  

A few other amazing facts : 

●Hundreds of millions, even billions of birds, migrate each spring and fall. Their migration occurs mostly at night. This is why we aren’t aware of the spectacle.

●Birds that are ill-equipped to land on water have evolved a special way to sleep during their multi-days long migratory journey. Unihemispheric sleep is a way for one half of the brain to nap while the other is awake. After a few seconds the resting half awakens and the other half takes a brief nap. This on and off cycling continues through the bird’s migratory journey across the water.  

●Long distance travel is sure to make a bird thirsty. When there is no place to find fresh water, as is the case migrating over the ocean, birds have overcome the problem by biochemically extracting water from their organ and muscle tissues.

Bar-tailed godwit, photo by Geoff White Photography.

Super Marathon Flyers 

One super marathon migrator is the bar-tailed godwit. According to Weidensaul, the shorebird travels some 7,000 miles, nonstop across the Pacific Ocean from West Alaska to New Zealand. The journey takes upwards of 11 days. In 2021, an adult, male set the world record for the longest distance made by continual flight of any land-based bird.The adult male Godwit flapped its wings for 239 hours covering 8,100 miles. This record was verified because by the bird which was outfitted with a solar-powered location tracker. 

Unlike human marathoners, the birds don’t carbohydrate load but they do prepare for their long distance journey by binge feeding. In Alaska godwits breed and nest along coastal and estuarine mudflats where they use their long beaks to feed on crustaceans, mollusks, and insects in the tidal mud. The birds eat enough to double their weight and fat composition. Weidensaul describes them as jiggly “like water balloons.” That’s not all that gets bigger. The godwit’s heart and flight muscles increase 50% by mass. It’s a wonder the birds can fly.  This size increase is offset by the fact that parts of the godwit actually shrink. Having eaten enough to build a surplus of fat for their migratory flight, the bird’s digestive system shrinks. When the birds reach New Zealand, the fat reserves depleted by the flight must be replenished. The gut is regrown and the bird can begin to store food energy. A few months later, the direction of the migration must be reversed and so the body, once again, prepares for the journey.    

The godwit is not the ultimate avian long distance record holder. That title belongs to the Arctic tern. Unlike the godwit, terns are seabirds. They are able to land on water. They feed by plunging into the ocean in order to catch small fish. Terns, however, for short periods each year spend time ashore. They seek land in the Arctic to establish their nests, lay eggs, and rear their chicks. Then they migrate south seeking summertime in Antarctica. They journey annually from the Arctic to Antarctica and back. Their pole to pole travels covers about 25,000 miles each year. The oldest recorded Arctic turn was at least 34 years old when it was recaptured in a banding operation in Maine. Do the math and you find that this individual probably travelled more than three quarters of a million miles during its life time. That’s a lot of frequent flier miles!

An albatross outfitted with a GPS tracker, Henri Weimerskirch

How do we know these things?

The science of studying birds, ornithology, is probably as old as our fascination with flight. Detailed, recorded observations are a more recent phenomenon. In the United States, recorded observations of individuals birds can be linked to the America naturalist and painter John James Audubon who in the early 1800s tied threads to birds’ legs. A century later scientists affiliated with the Smithsonian attached metal bands with identification numbers to the legs of herons found on the grounds of the National Zoo. If the herons were recaptured the bands identified the individual and data specific to the bird. This method of mark and recapture is still used. Mark and capture data is useful in the estimation of the abundance of a population as well as tracking the movement and fate of individuals.

Technology has enhanced our ability to gather population and individual data. Birds outfitted with electronic tracking devices, such as a light-level geolocator or satellite transmitters, provide real time and more detailed data. Weidensaul said that the Doppler radar stations operated by the National Weather Service to track precipitation and other meteorological events are also useful in tracking the migration of birds at night. Dual polarization weather radar has taken research to new heights (pun intended). Dual polarization radar provides sufficient detail to allow researchers to distinguish the size of birds from small birds to large ones and even to know which direction they are pointed, beak end or tail end.

In an interview on NPR Weidensaul said: “And because we have archived weather radar images going back to the beginning of the Doppler radar era in the 1980s, because we have so much computing power now to take these, you know, millions and millions and millions and millions and millions of radar images, we can actually look back in time and calculate how the number of migratory birds has changed over time. And so that’s one reason that we know - is that since 1970, we’ve lost about a third of North America's birds. About - almost 3 billion birds have disappeared from North America compared to the 1970s. And, you know, and that's not a guesstimate. That's based on solid radar data.”

What we don’t know

We know that most birds migrate after dark and now we have technological advances that provide us ways to keep ever more detailed watch over them. As a result we know a great deal about their amazing adaptations and life habits. Birds have developed an amazing array of adaptations for the demands of life on earth. The question now: Can they adapt fast enough to cope with the rapid changes brought about by humans and their technology?