If you wanted to find out where Barn Swallows migrate each year, it wouldn’t be any harder than a Google search. But believe it or not, no one knew where they went with certainty until the twentieth century, despite millennia of attempts to solve the puzzle. And despite all our technology, we’re still in the dark about a lot of migratory behavior!
Migration is the single largest natural phenomenon on earth. Birds, mammals, insects, fish–just about every taxonomic kingdom has examples of entire populations moving anywhere from 10 feet to 40,000 miles in a year. It’s not hard to imagine that even the earliest humans noticed that certain birds and animals disappeared for parts of the year. But to where?
In the fourth century BCE, the great Aristotle focussed his mind on this problem. It was supposed that large birds might be able to travel long distances, but smaller birds, like swallows, could not. Instead, he suggested that swallows hibernated in hollow trees in the winter, or even at the bottom of ponds. That view wasn’t disproved for over 2,000 years; despite all of humankind’s advancements and ingenuity, the flight of the swallow was simply untraceable.
One step toward understanding was the use of banding, whereby birds are captured and a small identifier is attached to a leg or wing. The bander then waits and prays that the bird is recovered, a hope that is unfulfilled more than 90% of the time. When a bird is recaptured, a window is opened–into its life–albeit a narrow one.
One of the first banders in North America was bird illustrator James Audubon. In the first years of the 1800s, Audubon tied a silver wire around the leg of an Eastern Phoebe and saw that the same bird returned to its nest each year. One hundred years later, systematic bird banding began in the United States, and became a federal program in the 1920s. Today there are 6,100 licensed banders in North America, who catch and band 1.2 million birds each year.
The use of electronics came to the fore in the sixties, and larger animals were affixed with radio tracking collars and monitored by researchers. These radio tags evolved to become increasingly smaller and lightweight, and radio signals were supplemented with both cellular and satellites. Today it’s becoming more common to track migration of birds like raptors. Today’s trackers can weigh as little as 0.1 gram (so light you might not feel it in your hand) and can be affixed to animals as small as butterflies. Aristotle would have been impressed!
Thanks to banding and other technology, we finally know where the swallow goes. We can sympathize with our ancestors, since it turns out swallows are some of the longest distance migrants, flying from places like Canada to Argentina or Britain to South Africa every year!
Yet, these technologies still rely on capturing an animal and banding it, often at significant expense. Even basic tracking tags can cost $200, and satellite tags are commonly in the thousands, not to mention the human time in the field.
Bioacoustics, another form of tracking, may solve that problem. Birds call when they migrate, and by setting out microphones we can track not just individual bird calls, but entire populations of bird calls as they fly overhead. With a large enough network of these devices, we’ll gain a picture of population sizes and movements that will be exponentially greater than anything we have today.
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