The correct option is C Small birds
In a single tree ecosystem that contains smaller birds, tree, insects and larger birds, smaller birds would be the second most abundant in number. Trees are the primary producers while insects and birds are the primary consumers. In a single tree ecosystem, all organism including smaller birds, insects, and larger birds depend on the primary consumer for their nutrition. Smaller birds will eat insects that result in decreasing the population of insects. Larger birds will eat insects and smaller birds both. Therefore, in a single tree ecosystem, larger birds would be the first most abundant in number, and smaller birds would be the second most abundant in number.