Using three layers of vegetation to maximize habitat.
Overstory, mid story and understory trees and vegetation provide different habitat opportunities for birds. This Cornell Lab of Ornithology poster demonstrates the importance of each layer. Birds need a variety of vegetation heights to maximize their opportunities for safe feeding and breeding. Canopy structure, like trees in different heights and shapes, mid-level vegetation, such as shrubs, and structure close to the ground in the form of rock piles, leaf litter, or ground cover all fulfill slightly different purposes. (Undated) Cornell Lab of Ornithology.