Really it depends on an number of factors, but around here, starting in late July and extending through late September many birds are on the move away from their breeding grounds. Some are merely altitudinal migrants, descending when the weather turns fowl in the mountains. But a large number undertake a twice yearly journey that can span continents and oceans. Raptors let thermals carry them much of the way, songbirds power through with the help of highways of wind, and seabirds harness the oceanic air streams. A few of the many reasons birds are on the move is to avoid harsh weather and often more accurately to take advantage of seasonal or episodic food sources that good weather brings, particularly in temperate clines. There are migratory birds in every place on earth and the reasons, methods, and extremities are as diverse as the species that practice.
This phenomenon is inherently mystical, fascinating, and curious for humans. After all, with the rare modern exceptions of hunter gatherers that follow seasonal food sources, we don’t migrate. However, for millennia those of us in the Northern Hemisphere noticed when our vibrant sprouts begin to push up, we begin seeing swallows again; by the time the major harvests began, the field grew quiet once more.
Ancient Greeks believed absent swallows burrowed underground.
These journeys often defy our concepts of reality. Discussions about the record holder long distant migrants shifted from the Arctic Tern (flying from the Arctic Circle to Antarctica), to the Bar-tailed Godwit (flying from the Alaska to New Zealand), to the Sooty Shearwater (Northern Hemisphere to Southern), and back again. The Bar-tailed Godwit is held to be longest non-stop migrant some flying about 7,000 miles in one go without any dilly dallying (they fly over water and cannot float!). Sooty Shearwaters and Arctic Terns have both been recorded in distances exceeding 30,000 miles. In 2006 Sooty Shearwaters broke the record for the longest distance animal migration at around 40,000 miles, making a figure 8 tour between North and South Pacific each migration. 2010 research results on Arctic Terns showed them travelings in excess of 40,000 miles in their year of travel. (all this knowledge is a result of bird banding and lightweight radio tags). Seabirds certainly make some of the most spectacular hauls and spend a lot of time preparing for it, allowing for around 50 percent of their body weight to store fat reserves.
Here in the Puget Sound, many of our migrants are songbirds bound for the Neotropics. They are the reason people buy Shade Grown Coffee, because they winter in places in Mexico or Central America that produce coffee and would otherwise cut down forests to grow it. Equally astounding is their travel, which may start as far as Alaska or Northern Canada or may be a bird that reared young in your backyard in Seattle, but will end in some instances as far South as Panama. Not only that, but it is done largely at night!
Although it seems odd that diurnal, typically terrestrial species are flying high in the sky at night, there are some very logical admissions it allows for. The normal predators, who would be almost impossible to avoid in high in the open during the day, are asleep. Secondly, although birds fuel up before they leave, stops are necessary for food and water which they cannot do at night. Finally, the most interesting part is that they are using the stars to navigate! We long pondered how many birds were migrating at night until a technique using an instrument called an Emlen Funnel was invented in 1966 (follow the link to learn how it works).
Go out on a still night during migration and you can hear the calls of migrating songbirds. In the city, even above the noise of urban life, they are audible. Residing in the suburbs or away from busy roadways, one can hear hundreds, even thousands, of individuals representing dozens of species (with a bit of practice you can start to differentiate). Some enthusiasts actually set up their own recording devices and use programs to sort and count what flew over each night. Cornell Lab of Ornithology embarked on research doing exactly this, but on a larger scale along the Eastern Seaboard allowing for studies both on reasons for nocturnal flight calls and on population dynamics.
While these birds are on the move, they face many threats, not the least of which is the human influence. Our massive buildings and lights confuse birds in nightly migration. New York City’s light kill an estimated 10,000 birds a year. We destroy habitat not only at both ends of travel, but demolish their stop overs along the way. In short, these amazing travelers not only have the sheer obstacles of the elements, predators, and distance to come up against, but humans as well. Research allows us to understand this phenomenon but it also helps us evaluate how we can alter our ways to benefit birds.
What brought this diatribe about? This week, meteorologist and northwest weather expert Cliff Mass , posted a blog entry with fantastic Doppler Radar images showing birds nocturnally moving over the Puget Sound area. There are a lot and they are moving south!
Migration is something we still don’t fully understand and this article is just the tip of the iceberg on this fascinating subject. If you are interested in learning more I’d recommend Scott Weidensaul’s popular account, Living On the Wind: Across the Hemisphere With Migratory Birds. We are luck to have some amazing technologies from sound recordings, radar imaging, and light weight radio tags to help us imagine an otherworldly behavior.