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 Antarctican 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..
No comments:
Post a Comment