BOOKS: Animal Migration
From ANIMAL PEOPLE, September 2009:
Animal Migration: Remarkable Journeys in the Wild by Ben Hoare
University of Calif. Press (2120 Berkeley Way, Berkeley, CA
94704), 2009. 176 pages, 200 color illustrations. $34.95,
Though recognized by humans for far longer than recorded
history has existed, there is still no universally accepted
definition of just what migration is.
“Animals make all kinds of different movements–short and
long, seasonal and daily, regular and once in a lifetime, highly
predictable and seemingly random,” explains Animal Migration author
Hoare in Animal Migration explores the often mysterious
migratory patterns of at least 50 different species of birds,
reptiles, amphibians and insects. Most migrate as a necessity of
survival, in search of food and water, mates, and/or safe places
to lay eggs. When threatened, they move to avoid predators.
Climate chance or bad weather may force migration, or migration may
be caused by a combination of factors.
Some migrations are repetitively predictable, such the
breeding season movements of sea turtles. Sea turtles’ soft-shelled
eggs need beaches that lie at the right angle for the turtles to drag
themselves out of the water and back. Turtles dig pits in the sand
that operate as incubators. The temperature of the sand determines
the gender of the hatchlings. Sea turtles make repeat migrations to
nesting sites where–until global warming began altering the
conditions–the sand temperature tended to remain within a range
ensuring gender balance.
Lemming migrations are by contrast unpredictable and sudden.
The “lemming migration” depicted in the Walt Disney documentary White
Wilderness (1958) was a notorious fake, in which Disney unwittingly
used freelance footage of Canadian voles being tossed over a
waterfall as “lemmings rushing into the sea. ” Yet lemming
irruptions [sudden surges of animals into new habitat] do occur, as
do irruptions of other rodent species. They may react to food
shortages or overcrowding. Some investigators believe disease
outbreaks are involved. As yet, no one really knows.
Migration can be punishing on the animals during their
journeys, which can be quite long. Monarch butterflies, for
instance, migrate up to 3,000 miles. Many migrating animals never
reach their destination.
“The majority of small land animals simply cannot afford the
energetic cost of migration,” says Hoare. They may be eaten by
larger animals along the way or succumb to hunger or fatigue. The
survivors tend to be those who have prepared themselves with
intensive feeding to boost their fat reserves. Birds replace old and
worn plumage, as newer feathers improve their aerodynamics.
Migration is almost always a group phenomenon. Often wind or
water currents funnel migrating birds, insects, turtles, and fish
into the same travel corridors even when they start and finish at
different points. Mixing and mingling along the way helps some
species to maintain genetic diversity.
Like humans on long trips, migrating animals rest as they
travel. For some there may be safety in numbers at resting points,
but migratory waterfowl are most vulnerable to hunters when they
congregate at lakes or ponds.
Among the largest migratory waterfowl assemblies on record
were gatherings of 1.2 million snow geese counted at Sand Lake,
South Dakota in April 1991, and 800,000 seen at the De Soto National
Wildlife Refuge on the Iowa-Nebraska Border in November 1995.
Migrating insects gathering to rest and feed may be perceived
and feared like the Biblical plagues of locusts. Locust plagues
still afflict arid regions in Africa and the Middle East. Insect
migrations have caused havoc in Australia and parts of Latin America,
and grasshopper plagues were harbingers of the Dust Bowl in the U.S.
during the 1930s.
Other insect migrations are easily seen, yet barely noticed
by anyone but entomoligysts–and, sometimes, by feasting birds.
“Swarms of butterflies and moths settle on trees and
buildings to roost overnight or until a spell of bad weather has
passed,” notes Hoare. This can be followed by unusual numbers of
insect-eating birds colliding with windows.
Inclement weather often disrupts migrations. Human habitat
modification is also increasingly problematic. Beachfront
development confuses hatchling sea turtles around the globe. The
adults already at sea find their traditional nesting beaches, but
instead of crawling toward the surf, guided by starlight,
hatchlings may head toward illuminated roads.
Within 10 or 20 years a newly lighted road may bring an end
to millions of years of turtle nesting migrations.
Hoare believes that seals and whales “can probably recognize
features on the seafloor,” which serve as roadmaps for their
migratory journeys, but polar bears often travel comparable
distances within the Arctic Circle through a landscape with few if
any semi-permanent features, completely dark for half the year.
Polar bear migrations tend to follow the movements of their major
prey, including ringed seals and walruses. Now rising temperatures
are shrinking the polar icecaps, shortening the bears’ feeding
More than 300 color pictures and maps complement Animal
Migration: Remarkable Journeys in the Wild. Hoare anticipates that
drought, floods, and hotter temperatures resulting from global
warming will disrupt the migratory habits of many more species than
polar bears. Animal Migration: Remarkable Journeys in the Wild
will be helpful in understanding the changes we may soon witness.
–Debra J. White