Some seabird colonies host thousands of nesting pairs of various species. Triangle Island, for example, the largest seabird colony in British Columbia, Canada, is home to auks, gulls, cormorants, shorebirds, and other birds, as well as some marine mammals. Many seabirds show remarkable site fidelity, returning to the same burrow, nest or site for many years, and they will defend that site from rivals with great vigour. This increases breeding success, provides a place for returning mates to reunite, and reduces the costs of prospecting for a new site. Young adults breeding for the first time usually return to their natal colony, and often nest very close to where they hatched. Individual nesting sites at seabird colonies can be widely spaced, as in an albatross colony, or densely packed like an auk colony. In most seabird colonies several different species will nest on the same colony, often exhibiting some niche separation. Seabirds can nest in trees (if any are available), on the ground (with or without nests), on cliffs, in burrows under the ground and in rocky crevices. Colony size is a major aspect of the social environment of colonial birds.