ACCORDING TO THE PASSAGE, WHICH FACTOR IS MOST IMPORTANT IN DETERM...

70.

According to the passage, which factor is most important in determining a pet’s behavior?

A. Its DNA

B. Its donor tissue

C. Its environment

D. Its surrogate mother

Passage C

Brood parasitism, an unusual practice among birds, involves one species laying its eggs in another species’

nest, leaving the host to raise the intruder’s young. For instance, female European Cuckoos lay their eggs

only in the nests of other birds. A cuckoo egg usually closely mimics the eggs of the host, one of whose

eggs is often removed by the cuckoo. The host may recognize the intruder’s egg and abandon the nest, or

she may stay and raise the young. Soon after the cuckoo hatches, if there are any host bird’s young in the

nest, the cuckoo will toss them out, using a scooplike depression in its back. The host parents are then

left to raise the young cuckoo.

Brown-headed cowbirds are another brood parasitic species. They have been known to parasitize over 200

other species of birds. Their eggs do not closely mimic host eggs, and they do not oust host eggs and young

from their nests. Instead, cowbirds tend to hatch earlier than hosts and grow faster, thus crowding out and

reducing the food intake of the host’s young.

Some host species have learned, however, to reject invader eggs. Scientists do not fully understand how

these rejector species have developed, or why some species still accept invader eggs even when the eggs

look different. Some scientists believe that acceptors are birds that do not want to risk damaging or

accidentally removing one of their own eggs when trying to eject an invader. Others believe that beak size

influences rejection, allowing birds with large beaks to eject invader eggs more easily. Still other scientists

claim that chance plays a big role in deciding which birds will be acceptors and which will be rejectors.