The Weirdest Ideas About Bird Migration
Each year, the ten-inch-long, 100-gram Arctic tern flies between 40,000 and 50,000 miles on a winding migratory course that takes it from its breeding grounds in and around the Arctic Circle, down to the tips of South America or Africa, even down to Australia, and all the way back. Tallying all of these journeys over the bird's thirty-year lifetime yields a collective trek equal to three round trips to the Moon! That such a tiny bird can accomplish such a monumental feat is patently ridiculous, and yet it is very real.
The notion that birds can migrate over vast distances must have seemed even more outlandish to early scientists. The Greek philosopher Aristotle was one of the first to grapple with the mystery of birds' seasonal disappearances and reappearances. As zoologist Lucy Cooke recounted on a recent episode of the Infinite Monkey Cage, the sage thinker considered three options: birds migrate to far-off lands, they transform into other birds, or they hibernate. Aristotle ruled out migration rather quickly, thinking it the most absurd. On transformation, he thought more deeply.
"Frequently one species would arrive from the north just as another species departed for more southerly latitudes," Frederick C. Lincoln wrote in the Fish and Wildlife Service's circular on bird migration. "From this [Aristotle] reasoned the two different species were actually one and assumed different plumages to correspond to the summer and winter seasons."
Aristotle mused that redstarts might annually morph into robins and garden warblers would change into blackcaps and vice versa.
His hypothesis that birds hibernate gained the most traction, however, persisting in learned discussions for well over 2,000 years.
"At the beginning of the Royal Society, a major point of debate was whether swallows hibernated underwater like fish because some obscure Swedish priest had said that he'd seen fishermen pulling swallows out of rivers," zoologist Lucy Cooke said.
That Swedish priest, Olaus Magnus, was actually a bishop, and in 1555 he wrote extensively on how swarms of swallows winter in the soft, unfrozen muds of shallow rivers and lakes. He wasn't the only one to make such claims. According to Lincoln, a few naturalists even recorded that fishermen in northern waters sometimes hauled in mixed catches of fish and hibernating swallows. This misinformation lasted all the way into the late 1800s, when American ornithologist Dr. Elliott Coues listed the titles of 182 papers dealing with the hibernation of swallows (p. 17) in his popular book Birds of the Colorado Valley. Though skeptical, Coues wasn't sure what to make of the claim.
"I have never seen anything of the sort, nor have I ever known one who had seen it; consequently, I know nothing of the case but what I have read about it. But I have no means of refuting the evidence, and consequently cannot refuse to recognize its validity... I cannot consider the evidence as inadmissible, and must admit that the alleged facts are as well attested, according to ordinary rules of evidence, as any in ornithology."
Apart from a single species, the common poorwill, birds do not hibernate, and the poorwill definitely does not do it underwater. At the southern edge of its range in the western United States, the poorwill nestles itself inside piles of rocks or a rotten log and slows its metabolic rate down by 90% or more for as long as 85 days.
Harvard College educator Charles Morton may have been one of the original scientific thinkers to nudge biologists back in the right direction, albeit in quite a fantastical way. In a paper published around 1694, Morton correctly noted that changes in environmental conditions and food availability prompted birds to migrate and smartly reasoned that underwater hibernation would inhibit their ability to breath. However, he went on to insist that birds didn't migrate to another hemisphere on Earth, but actually to the Moon!
Morton apparently got a little overexcited and obviously fell victim to poor reasoning, but hey, maybe if we outfit arctic terns with spacesuits and launch them from orbit, they might just make it to the lunar surface!