A limiting factor is something in a ecosystem that limits the growth of a species. In this case, cowbirds are limiting factors to wood thrushes because they lay eggs in the nests of wood thrushes, which is called "nest parasitism." A decrease in the carrying capacity—the number of living organisms that a region can support (without environmental degradation)—of wood thrushes could be caused by a decrease in the insect population size. The less insects, the less food. With less resources, the carrying capacity will decrease for wood thrushes. If the cowbird population were decreased, or the space or number of nesting sites were increased, the carrying capacity would increase, instead.
Answer: B