The wood thrush makes its nest in woodlands in southeastern Canada. As patches of woods become ...

The wood thrush makes its nest in woodlands in southeastern Canada. As patches of woods become smaller, and with more of their perimeter exposed, increasing numbers of wood thrush nests have eggs laid in them by the brown-headed cowbird. Cowbirds live in open fields and are nest parasites; the female cowbird flies up to an unattended nest, quickly lays an egg in another bird's nest, and leaves the host parents to raise their "adopted" young. Wood thrushes are decreasing in numbers because ________.
A) habitat fragmentation makes it easier for cowbird parasitism to occur
B) cowbirds are an invasive species that is rapidly increasing
C) climate change is reducing nest site availability
D) overharvesting in the open fields has driven the cowbirds into the woodlands
E) pollution of the open fields has caused a change in cowbird behaviour