https://nestvui.com/yen-hu-chung-san/
When you think of a bird’s nest, you likely envision a cup-like structure made out of sticks, grass and other materials. However, some species build a different type of nest, called a cavity. Cavity nesters do not construct their own open-cup nests, but instead take advantage of naturally occurring holes in trees, riverbanks, cliffs or human-made structures (like bird houses). The birds often have extra protection from predators and harsh weather conditions as well. The majority of North American bird species that live in woodlands are cavity neste