Bird migration is one of the most fascinating phenomena of the natural world. Of course, I may be a little biased in my judgement. However, it is hard to argue that migrating birds are spectacular, weaving invisible threads all over the world’s skies and connecting all habitats and ecosystems of the Earth.
Imagine, for an instant, being on a windswept, treeless tundra in Alaska. It is Fall, the days are getting shorter and temperatures are dropping. You are a Bar-tailed Godwit, a long-legged, long-billed, 340g, shorebird, feeding non-stop to get nice-and-fat before you embark on your long journey. After several weeks, you have increased your body fat by more than 50% and are waiting for a strong depression, which would sweep you off with a strong tailwind for your odyssey. And it is an odyssey, worthy of Greek mythology, even though you do it every Fall: you will embark on an oceanic crossing, aiming for the Antipodes, that is, New Zealand, in a non-stop, 10,400km flight, which will take more than eight days (and nights). Amazingly, for us humans, land-bound with feet of clay, you make your way from the northern reaches of the Earth all the way to the Southern Hemisphere. The navigational precision required to accomplish this feat is truly remarkable.
Thus, Alaska and New Zealand are connected by one Godwit, flying back-and-forth every year.
It is certainly a striking example but there are many more. At Cabot Head Research Station, we have been seeing and banding Blackpoll Warblers lately, a 12g songbird of the boreal forest. They do their own oceanic journey: after congregating on the shorelines of New England and the Maritimes, they take off over the sea, in a southeast direction – with the aid of tailwinds – and fly away, until they reach the trade winds. Then, they turn southwest, heading to South America, where they will spend the winter.
To accomplish these migratory achievements, birds go through some major physiological changes. At migration time, birds exhibit restlessness, or Zugunruhe as it was first described by German scientists using caged birds: it is an increase of activities, such as wing-whirring, hopping and perching. But the major physiological change necessary for migration involves the accumulation of fat and other body reserves to sustain the bird and fuel its flight, since most birds cannot forage while flying. The urge to feed is called hyperphagia and is essential for acquiring the needed fuel reserves.
We sometimes observe the capacity of putting on weight when we recapture birds. For example, this year, two Red-eyed Vireos banded on September 2nd were recaptured five days later. Both had increased their fat score and weight: one vireo went from 16.3g to 18.4g, a 12% increase, whereas the other one went from 15.1g to 18.5g, a 29% increase!
The need to replenish fat reserves to successfully complete their migration illustrates the importance of good habitats along the routes of the migrant birds. It is essential – read, vital! – that birds find appropriate stopover sites to rest and eat, and eat, and eat some more, along the way. The connections migratory birds make across the world are thus not just between “breeding” and “wintering” grounds. It also includes all the places in between, like a string of pearls, some pearls bigger than others but all equally important.
Because, in the end, very few birds are like the Bar-tailed Godwits, flying non-stop for days on end with no need to stop at various “pearls”. And even this incredible bird does need the in-between places: in spring, its migration follows a more westerly course, along the Asian Pacific Rim, with multiple stops on mudflats and wetlands.