SAT Reading Practice Test 3

This passage is adapted from Thor Hanson, Feathers. ©2011 by Thor Hanson. Scientists have long debated how the ancestors of birds evolved the ability to fly. The ground-up theory assumes they were fleet-footed ground dwellers that captured prey by leaping and flapping their upper limbs. The tree-down theory assumes they were tree climbers that leapt and glided among branches.

At field sites around the world, Ken Dial saw a
pattern in how young pheasants, quail, tinamous,
and other ground birds ran along behind their
parents. “They jumped up like popcorn,” he said,
5 describing how they would flap their half-formed
wings and take short hops into the air. So when a
group of graduate students challenged him
to come up with new data on the age-old
ground-up-tree-down debate, he designed a project
10 to see what clues might lie in how baby game birds
learned to fly.
Ken settled on the Chukar Partridge as a
model species, but he might not have made his
discovery without a key piece of advice from the local
15 rancher in Montana who was supplying him with
birds. When the cowboy stopped by to see how
things were going, Ken showed him his nice, tidy
laboratory setup and explained how the birds’ first
hops and flights would be measured. The rancher
20 was incredulous. “He took one look and said, in
pretty colorful language, ‘What are those birds doing
on the ground? They hate to be on the ground! Give
them something to climb on!’ ” At first it seemed
unnatural—ground birds don’t like the ground? But
25 as he thought about it Ken realized that all the
species he’d watched in the wild preferred to rest on
ledges, low branches, or other elevated perches where
they were safe from predators. They really only used
the ground for feeding and traveling. So he brought
30 in some hay bales for the Chukars to perch on and
then left his son in charge of feeding and data
collection while he went away on a short work trip.
Barely a teenager at the time, young Terry Dial
was visibly upset when his father got back. “I asked
35 him how it went,” Ken recalled, “and he said,
‘Terrible! The birds are cheating!’ ” Instead of flying
up to their perches, the baby Chukars were using
their legs. Time and again Terry had watched them
run right up the side of a hay bale, flapping all the
40 while. Ken dashed out to see for himself, and that
was the “aha” moment. “The birds were using their
wings and legs cooperatively,” he told me, and that
single observation opened up a world of possibilities.
Working together with Terry (who has since gone
45 on to study animal locomotion), Ken came up with a
series of ingenious experiments, filming the birds as
they raced up textured ramps tilted at increasing
angles. As the incline increased, the partridges began
to flap, but they angled their wings differently from
50 birds in flight. They aimed their flapping down and
backward, using the force not for lift but to keep
their feet firmly pressed against the ramp. “It’s like
the spoiler on the back of a race car,” he explained,
which is a very apt analogy. In Formula One racing,
55 spoilers are the big aerodynamic fins that push the
cars downward as they speed along, increasing
traction and handling. The birds were doing the very
same thing with their wings to help them scramble
up otherwise impossible slopes.
60 Ken called the technique WAIR, for wing-assisted
incline running, and went on to document it in a
wide range of species. It not only allowed young
birds to climb vertical surfaces within the first few
weeks of life but also gave adults an energy-efficient
65 alternative to flying. In the Chukar experiments,
adults regularly used WAIR to ascend ramps steeper
than 90 degrees, essentially running up the wall and
onto the ceiling.
In an evolutionary context, WAIR takes on
70 surprising explanatory powers. With one fell swoop,
the Dials came up with a viable origin for the
flapping flight stroke of birds (something gliding
animals don’t do and thus a shortcoming of the
tree-down theory) and an aerodynamic function for
75 half-formed wings (one of the main drawbacks to the
ground-up hypothesis).