The number of entrances may be important too. Gibbons says when studying the hollow requirements of forest ecosystems, it cannot be assumed that unoccupied hollows are unused. Their use may be seasonal, or multiple. Greater gliders in south-east Queensland have been found to use three or four differe nt dens a month and from four to 18 over 10 months. Yellow-bellied gliders also change dens frequently.
Territorial behaviour is exhibited by a number of hollow-dependent species.
Some marsupials and birds may defend vacant hollows from other species. Forest owls rely on trees with hollows as nesting sites, and also because, for some species, their major food source is arboreal marsupials. It has been suggested that the powerful owl may consume 250-300 possums annually. A forested area of about 1000 he ctares is needed to satisfy such an appetite. Gibbons says ensuring continuity in the hollow resource over time is critical to effective forest wildlife management.