199
See also: Evolution by natural selection 24–31 ■ Global warming 202–203
■ The Keeling Curve 240–241 ■ Ozone depletion 260–261 ■ Spring creep 274–279THE LIVING EARTH
Receding glaciers
and bird migrationWhen the last glacial period
began to end, around 26,500
years ago, Earth was much
colder than it is today. Much
of North America and northern
Eurasia was covered with ice
sheets. The environment was
so harsh that most birds
tended to live in subtropical
and tropical regions where
there was more food.
As temperatures began
rising, the ice sheets started
to shrink, uncovering a new
landscape. Ice-free ground
and short, wet summers were
ideal for insects, and birds
began to move in, too, to
take advantage of this food
supply. When days got shorter
in fall, some birds stayed on
for the winter, but others
returned south.
The distances flown by
birds returning to their homes
grew longer as the ice sheets
retreated farther, eventually
developing into long-distance
spring and fall migrations
between the tropics and
northern latitudes. Common
birds that undertake the
journey include swallows,
warblers, and cuckoos.A male Baltimore oriole perches
on a tree fern in Costa Rica. The
species flies north to breed in
March and returns to the tropics
in August or September.Glaciers converge on Piz Argient,
a mountain in the Swiss Alps. Like
others in the Alps, these glaciers were
once much more extensive than they
are now, and they continue to shrink.glaciers in 1840. The same year, he
visited geologist William Buckland
in Scotland to investigate glacial
features there, prompting Scottish
glaciologist James Forbes to begin
similar research in the French Alps.
Some quarters, such as the
Catholic Church, still argued that
glacial striations had been caused
by a great flood or that large silt and
rock deposits had been transported
by icebergs swept along by the flood.
From the 1860s, however, there was
wide support for Agassiz’s glaciation
theory and the idea that glaciers in
the Swiss Alps and Norway had
once extended much further. It was
also accepted that a sheet of ice
had once spread across Europe, and
south from the Arctic through much
of North America, with catastrophic
implications for plants and animals.
By the late 1800s and early
1900s, as more expeditions to both
Greenland and Antarctica were
undertaken, it became known that
both areas were still covered in ice.Aerial surveys in the 1920s and
1930s confirmed the extent of their
vast ice sheets—now defined
as areas of glacier ice exceeding
19,300 sq miles (50,000 sq km); ice
caps, such as Iceland’s Vatnajökull,
are smaller.
Further evidence revealed that
there had not been one single ice
age, but at least five major ice ages
in Earth’s long history. The most
recent, the Quaternary Ice Age,
began 2.58 million years ago and is
ongoing. In the last 750,000 years,
there have been eight ice advances
(glacial periods) and retreats
(interglacial periods). During the last
glacial period, which ended 10,000–
15,000 years ago, ice sheets were up
to 2½ miles (4 km) thick, and the sea
level was 390 ft (120 m) lower. ■US_198-199_Ancient_Ice_Age.indd 199 12/11/18 6:25 PM