determining what bald eagles eat. The best approach
is to observe the eagles and identify the prey they
capture, but this is often difficult. Most studies use
an indirect method for determining dietary habits. The
eagle's diet can be reconstructed either by examining
food remains at the nest and feeding perches or by
analyzing regurgitated pellets, which contain the
undigested portions of the food. Pellet analysis
is, however, a poor research technique because
fish remains are either totally absent from or
underrepresented in pellets, since fish are more
easily digested than other foods.
The vast diversity of prey that eagles exploit is
exemplified by the variety of prey remains found at
their nests or under their feeding perches. Also,
eagles will feed on prey of any size. Bald eagles
are known to eat small rodents, but they also dine on
beached whales. Their food habits can change daily
or seasonally and from one location to next, and
their varied foraging tactics mean that their diet will
also be diverse. When eagles steal prey from other
predators, for example, they exploit a whole new range
of food sources. Perhaps the only restriction the
eagle faces is in the location in which it seeks prey.
Eagles require large, open expanses of water or land
for foraging and feeding.
Bald eagles feed primarily on fish, birds,
and mammals. Using information from 20 studies
of nesting eagles in North America, an overall
average diet can be calculated: 56 percent fish,
28 percent birds, 14 percent mammals, and 2 percent
miscellaneous sources. In other words, nesting
eagles eat twice as many birds as mammals, but
twice as many fish as birds.
When a choice is available, bald eagles invariably
select fish over other prey. Preference tests, in which
fish, bird, and mammal carrion were placed at feeding
areas or provided to captive birds, showed that fish
were chosen most often, followed by birds, then
mammals. Although fish may be preferred, prior
experience can greatly influence a bird's choice.
An eagle that has been eating a particular item will
likely continue foraging for it as long as it remains
available.
245. According to the passage, what is
one effect of an eagle stealing prey
from other predators?
A. It reduces the time required for
hunting.
B.The eagle is able to hunt close
to its nest.
C.Other predators are driven
away.
D.The variety of food available to
the eagle is increased.
答案是D，我看了四個答案，還是選不出來..
，我猜可能是從紅線這句話去推敲，但我還是不太懂為啥要選D，可能是文法理解力太差....
