A number of animals quite enjoy a snack of eggs, chicks and even adult birds, stowed away in tree hollows or nest boxes. Other species, like smaller possums (such as Sugar Gliders), may only use a nest box that cannot be accessed by larger animals such as Brushtail Possums, probably for safety reasons.
In some areas there is significant competition by introduced species such as Starlings, Indian Mynas, or even by Sugar Gliders introduced to Tasmania, for tree hollows, displacing local native animals from their homes.
Unwanted nest box competitors can also include common native animals, particularly where there are simply not enough hollows to go around, because of widescale loss of old-growth forests and mature hollow-bearing trees.
Predators of nest box occupants include a number of native species, and also introduced species. In regions where tree hollows are scarce, animals may take any hollow they can, regardless of risk of predation or eggs being raided because of a sub-optimum (such as very visible) location or larger than desired entrance size.
Nest box predators include: Brushtail Possums, Sugar Gliders, Ravens, Currawongs, free ranging cats and foxes (yes, they can climb!).
Nest box competitors include: Brushtail Possums, Ringtail Possums, Sugar Gliders, Indian Mynas, Starlings, Rainbow Lorikeets.
Here are a series of links to pages with information on common predator or competitor species, and methods to improve nest box success for the target species. I have included those predator and competitor species that I am aware of, please let me know of any others not included here, and I will add information about them in the future.