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While young martins and swallows return to their nests after their
first flights, young swifts do not. As soon as they tip themselves free of the
nest hole, they start flying, and they will not stop flying for two or three
years, bathing in rain, feeding on airborne insects, winnowing fast and low to
scoop fat mouthfuls of water from lakes and rivers.

To avoid heavy rain, which makes it impossible for them to feed, swifts with
nests in English roofs will fly clockwise around low-pressure systems, traveling
across Europe and back again. They love to assemble in the complicated, unstable
air behind weather depressions to feast upon the abundance of insects there.

On warm summer evenings, swifts that aren’t sitting on eggs or tending their
chicks fly low and fast, screaming in speeding packs around rooftops and spires.
Later they gather higher in the sky, their calls now so attenuated by air and
distance that to the ear they corrode into something that seems less than sound,
to suspicions of dust and glass. And then, all at once, as if summoned by a call
or a bell, they fall silent and rise higher and higher until they disappear from
view. These ascents are called vespers flights, or vesper flights, after the
Latin vesper for evening.

Swifts weren’t just making vesper flights in the evenings. They made them again
just before dawn. Twice a day, when light levels exactly mirror each other,
swifts rise and reach the apex of their flights at nautical twilight.

Adriaan Dokter, an ecologist with a background in physics, has used Doppler
weather radar to find out more about this phenomenon. He and his co-authors have
written that swifts might be profiling the air as they rise through it,
gathering information on air temperature and the speed and direction of the
wind. Their vesper flights take them to the top of what is called the convective
boundary layer. The C.B.L. is the humid, hazy part of the atmosphere where the
ground’s heating by the sun produces rising and falling convective currents,
blossoming thermals of hot air; it’s the zone of fair-weather cumulus clouds and
everyday life for swifts. Once swifts crest the top of this layer, they are
exposed to a flow of wind that’s unaffected by the landscape below but is
determined instead by the movements of large-scale weather systems. By flying to
these heights, swifts cannot only see the distant clouds of oncoming frontal
systems on the twilit horizon, but they can also use the wind itself to assess
the possible future courses of these systems. What they are doing is forecasting
the weather.

And they are doing more. As Dokter and his colleagues write, migratory birds
orient themselves through a complex of interacting compass mechanisms. During
vesper flights, swifts have access to them all. At this panoptic height, they
can see the scattered patterns of the stars overhead, and at the same time they
can calibrate their magnetic compasses, getting their bearings according to the
light-polarization patterns that are strongest and clearest in twilit skies.
Stars, wind, polarized light, magnetic cues, the distant stacks of clouds a
hundred miles out, clear cold air, and below them the hush of a world tilting
toward sleep or waking toward dawn. What they are doing is flying so high that
they can work out exactly where they are, to know what they should do next.
They’re quietly, perfectly, orienting themselves.

Thanks to Belinda and Gary Solomon.
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