Over time, certain areas of the forest ecosystem in the United States have decreased in size. Also, through a process called fragmentation, the ecosystem has been broken up into several isolated areas. One result of fragmentation is that more parts of the forest ecosystem now lie exposed to adjacent open lands.

Cowbirds are a species of bird that live in these open lands next to forests. Cowbirds lay eggs in the nests of other bird species, called host species. When a cowbird egg hatches, the unsuspecting host parent raises the cowbird as offspring. Often, a cowbird hatchling outcompetes or kills all the host species' offspring that share the nest. This results in a decrease in the host species' population and an increase in the cowbird population. Based on this information, how does forest fragmentation affect species of forest birds? A. It benefits their populations by protecting their nests from cowbirds. B. It threatens their populations by exposing their nests to more cowbirds. C. It threatens their populations by protecting their nests from cowbirds.