This 16th century woodblock shows fisherman pulling a big netload of hibernating swallows
from a frozen-over lake. If you haven’t heard about how swallows hibernate through the winter
at the bottom of lakes, it’s because they don’t. But, for thousands of years, hibernation
was one of the leading theories to explain where birds went between fall and spring.
Another theory was that the birds left entirely and flew far away for the winter — we call
this "migration" — but they didn’t have a clue where the birds would go. For example,
a pamphlet from 1703 suggested that they went to the moon.
The first real clues about where migratory birds actually go during the winter — hint:
it’s not the moon or the bottom of a frozen lake — came around 1900, thanks to a Danish
teacher’s technique of attaching marked aluminum rings to birds’ legs and then re-releasing
them. Each recapture or sighting of a banded bird put a dot on the map, and soon, long-distance
earthly migrations were confirmed when a White stork that had been banded in Hungary was
found dead in South Africa. But banding can only tell researchers about single points
along a bird’s migratory path — not what happens BETWEEN those points.
More recently, researchers finally started to get a better view — a bird’s-eye view,
in fact — of these annual migrations, when a bald eagle in Maryland was captured and
fitted with a transmitter powerful enough to send signals to a pair of orbiting satellites.
Satellite tracking revealed details of some remarkable migrations, like the bar-tailed
godwit’s annual flight from Alaska to New Zealand, during which the bird covers 11,000
km in about eight days without a single stop. But there’s a serious limitation to satellite
tracking devices: even with modern technology, transmitters with enough oomph to send signals
to satellites are still far too heavy for small songbirds.
A slight improvement is to use gps recorders, which can be smaller because they receive
rather than send signals to satellites — but they’re still too heavy for the smallest birds.
Luckily, scientists have been clever enough to realize they don’t need satellite tracking
at all! Instead, we can fit birds with a tiny light-level recorder, clock, and memory chip,
which together weigh as much as a raisin. Lightweight light-recorders don’t broadcast
so we need to recapture the birds to get the data, but we can then use ancient navigation
methods to reconstruct the bird’s daily location over the course of its journey: the length
of each day is an indicator of latitude, and the time halfway between sunset and sunrise
(that is, noon) is an indicator of longitude.
These clever geolocators have shed light on the world’s speediest migration: the Great
snipe, which weighs about 170 grams (half a can of soda-pop), high-tails it from Sweden
to Central Africa in just three days, averaging 95 km per hour.
Another marathon migrator, the Arctic tern, has long been credited with the longest migration
for its annual round-trip flight between the Arctic and Antarctic—an estimated 40,000
km. But recent data from light-level geolocators show that terns actually travel more than
twice as far each year, possibly to take advantage of prevailing winds.
This means that arctic terns can rack up over 2 and a half million kilometers of flight
in a lifetime — enough for three round trips to the moon. But as far as we know, they haven’t
actually made it there yet.