All species of birds lay eggs, and most build a nest to hold their eggs. A few birds don't build a nest, incubate(孵化) eggs, or care for young at all. They leave the responsibility for raising their babies to other species. This breeding strategy is called brood parasitism(巢寄生), and it's done by birds called brood parasites.
Fewer than 1% of all birds are brood parasites. Over most of North America, the only brood parasite is the brown-headed cowbird. It makes sense for the brown-headed cowbird to be a brood parasite when you consider this bind's history. Long ago, its range was fairly restricted to the short grass prairies of the Great Plains,where it lived on seeds and insects if found on disturbed soil. How did the soil get disturbed? Big, heavy bison hooves broke up the prairie nod(草皮). When cowbirds followed the bison, they could find food on the ground, some even in and around the bison droppings, which held partly-digested seeds and attracted insects.
This worked great most of the year. But during the breeding season, the cowbirds would have had a problem if they nested like other birds. Bison herds move about constantly. Imagine what would happen if a cowbird started nesting and suddenly the bison left! Without "help" from the bison, how could she possibly feed her babies or herself if she stayed? Being a brood parasite, she stayed with the bison, knowing her babies were safe in the nests of birds who found food in other ways.
Female cowbirds will stay quietly in the tops of shrubs or trees, watching for nest-building activities. Once the cowbird locates the nest, she usually removes one egg. Many nesting birds don't even seem to notice a cowbird egg among their own eggs. Once the cowbird nestling hatches, the foster parents raise the nestling as if it were one of their own.
Cowbirds remain with their foster parents for about 13 days after they leave the nest,following and begging from adults who are often very much smaller than the baby cowbirds! But after about two weeks, the babies go off on their own, and somehow instinctively associate with other cowbirds.
4. What do we learn about the brown-headed cowbird from the second paragraph?
A.It has always been a brood parasite. |
B.It has always lived a mobile life. |
C.It might build their own nests long ago. |
D.It used to have strong ability of adjustment. |
5. What's the relation between the cowbird and the bison?
A.They depend on each other for everything. |
B.The cowbird largely depends on the bison. |
C.They have to move together. |
D.They compete against each other for survival. |
6. The cowbird has become a brood parasite because________
A.it has to follow the bison all the time |
B.it has lost its skill of building nest |
C.it can't find proper places to build nests |
D.it can't get food from the bison for its young |
7. The cowbird removes one egg from the nest probably to_________
A.make space for its own egg |
B.reduce competitors for its young |
C.reduce the burden of the nesting birds |
D.fool the nesting birds |