The wood thrush makes its nest in woodlands in the northern and eastern united states. as patches of woods become smaller, 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 it, 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) climate change is reducing nest site availability.
c) cowbirds are an invasive species that is rapidly increasing.
d) pollution of the open fields has caused a change in cowbird behavior.
e) overharvesting in the open fields has driven the cowbirds into the woodlands.