Inquilinism is a form of commensalism in which one species uses another organism, its body, or its constructed space as shelter or a place to live without significantly harming it. A bird nesting in a tree gains support and a protected site for eggs and young, while the tree is ordinarily treated as unaffected. Phoresy would require the tree to transport the bird, and metabiosis usually involves using a structure or environmental modification after its producer has finished with it, such as a hermit crab occupying an abandoned snail shell. Parasitism would require a net cost to the tree caused
Ref:
Ecology: From Individuals to Ecosystems, Begon et al., 5th Ed., Ch. 8-13