Approximately 15 percent of Australia’s terrestrial vertebrate fauna species are dependent on tree hollows for either shelter or breeding.¹ This includes arboreal or tree dwelling mammals (possums and gliders), bats, birds (parrots, cockatoos and owls), arboreal reptiles (skinks and tree snakes) and amphibians (tree frogs). In Australia there are approximately 114 different species of birds, 83 mammal species, 79 reptile species and 27 amphibian species that use hollows for nesting and/or shelter. Eucalypts take at least 120 - 200 years to form hollows therefore retention of hollow bearing trees is extremely important for hollow dependant fauna.

The distribution of tree hollows throughout the landscape is highly dependant on the type of forest naturally occupying an area and the long term land use and management of that area. As a result of deforestation, land clearing and fragmentation, tree hollows are being removed from the natural environment. This is having a direct impact on native hollow dependent wildlife.

The abundance of tree hollows is decreasing due to the effects of:

• vegetation clearing.
• fire wood collection.
• bushfires.
• increased tree fall in disturbed areas.
• low tree recruitment due to clearing of understorey vegetation.

This loss of tree hollows has significant effects on the fauna biodiversity of bushland areas.

The community and private landholders can get involved, to improve the habitats for native animals by preserving hollow trees and erecting nest boxes.  Nest boxes can play a vital role in species conservation in areas where tree hollows have been removed or are limited.

Lifecycle of a Tree Hollow

• Tree hollows large enough for occupation by vertebrate fauna generally do not form until the tree is at least 120 years old.
• Hollows are formed by the decay of heartwood in the trunk or branches and subsequent damage of the tree which exposes the decayed heartwood.
• Heartwood is decayed by fungi and insect attack.
• Termites enter the trunk heartwood, through a damaged trunk or root system, and excavate the heartwood of the trunk and branches.
• Branches or the main trunk may snap off during storms or under their own weight. The exposed decaying heartwood then decays more rapidly or disintegrates to form hollows.
• Hollow formation can be accelerated through excavation by parrots, cockatoos and some arboreal mammals.
• Continued fall of branches and hollow limbs provides habitat for ground dwelling fauna or aquatic fauna if these branches fall into wetlands or creeks.
• Hollows may persist in trees for several hundred years after initial formation before the tree is destroyed by bushfire or falls to the ground under the natural forces of age and gravity.
• Tree hollows can be used by many different species throughout its lifetime.

Causes of Decline in Hollow Bearing Trees

• Hollow bearing trees are unevenly distributed across vegetation communities. Higher productivity areas (good quality soils and favourable moisture regimes) support more hollow bearing trees than drier areas of low soil fertility.
• Persistence of tree hollows in an area is influenced by:
• history of logging.
• pastoral and agricultural practices.
• bushfire regimes.
• altered climatic and storm regimes.
• altered recruitment potential due to changed land use and land management practice.
• tree felling for firewood collection.
• tree felling for urban amenity purposes.

Protection for Hollow Bearing Trees

The ecological significance of hollow bearing trees and the impact on biodiversity resulting from the loss or decline in numbers of hollow bearing trees has been recognised in both the Threatened Species Conservation (TSC) Act, 1995 and the Native Vegetation Act, 2003.

• The ‘Loss of Hollow bearing Trees’ has been listed as a key threatening process under the TSC Act.
• The ‘Removal of Dead Wood and Dead Trees’ has been listed as a key threatening process under the TSC Act.
• Native trees, including hollow bearing and dead trees in non-urban areas are protected under the provisions of the Native Vegetation Act.
• Within the Central Coast area trees located in urban areas are protected under the provisions of the various Councils Tree Preservation Order. To find out more go to Gosford City Councils Tree Works Information.
  

Use of Nest Boxes

• All possible options to retain a tree with hollows should be taken before considering nestboxes.
• The use of nest boxes as a replacement for tree hollows lost during vegetation clearing is becoming more common. Nest boxes are generally constructed of durable timber in a variety of sizes to suit a range of possible fauna species.
• While many fauna species use nest boxes for shelter and breeding purposes they should not be considered as a complete substitute to retained tree hollows.
• Nest boxes can be designed and located to attract specific fauna such as micro-bats, rosellas, lorikeets, cockatoos, gliders or possums and even native bees.
• In residential areas or urban parklands nest boxes will often be utilized by fauna species which are more adaptable to disturbed areas (such as kookaburras, possums, parrots and cockatoos) and exclude other less dominant species (such astree creepers, kingfishers, micro-bats and gliders).
• Nest boxes should only be installed in trees by professional arborists who can ensure that the nest box is installed in a manner that does not affect the health of the tree.
• Nest boxes should be installed high up in the tree to reduce the chances of predation of occupying fauna by cats and goannas.
• The size of the entrance hole of the nest box generally determines the type of fauna that will use a nest box. Micro-bats and small gliders can enter small holes or gaps (20 – 30mm diameter) while Brushtail possums, some parrots and owls require large entrance holes (100mm diameter).
• The depth of the box may also be an important factor; large birds such as owls may need a hollow up to 1m deep. The height in the tree where the nest box is positioned should also be considered.
• Nest boxes should contain small drainage holes at the base and 20-30mm of hardwood sawdust placed in the box. Nesting material is not required as this will be brought in by the fauna using the nest box.

Monitoring your Nest Boxes

To ensure your nest boxes are being used for the desired purpose and are successful, it is a good idea to periodically monitor your nest boxes once erected.  Things to look for include:

• Chew marks and scratches around the entrance is a good indication that they are being utilised.
• Feral animals should be discouraged from using nest boxes.¹
• Damage by storms or falling branches.
• Invasion by ant and bee colonies.¹

Keeping Introduced Species Out of Nest Boxes

There are some nest boxes designed to keep out introduced animals, like the Indian Myna, Starlings and European Bees to varying success, however all nest boxes should be monitored regularly for habitation by introduced species. There are a few things you can do to keep introduced species out of your nest box.

• Make sure they are far enough off the ground so they are less likely to be disturbed by dogs, cats and children.
• Some nest boxes, especially those for Rosellas, should be constructed with a baffle in front of the entrance to prevent entry by Mynas and Starlings.
• Watch nest boxes for colonisation by introduced species. Contact a pest controller for eradication if an introduced (feral) species such as bees occupies the nest box.
• Goannas and Diamond Pythons may be attracted to nest boxes. This, however, is a natural process.
• Climbing of the tree or inspection of the nest box using a ladder for support is not recommended for safety reasons and occupying fauna may also be disturbed by inspections. A better option may be to install a small surveillance camera or have a fauna specialist check the boxes periodically.

Useful Web links

• Flora for fauna - www.floraforfauna.com.au
• Birds Australia - www.birdsaustralia.com.au
• Department of Primary Industries (VIC) - www.dpi.vic.gov.au
• Environmental Protection Authority (QLD) - www.epa.qld.gov.au
• Flora for fauna. www.floraforfauna.com.au.
• State Forests of NSW Installing a Nest Box – Forest Facts www.forest.nsw.gov.au.

Further reading

• Gibbons, P and Lindenmayer, D (2002) Tree Hollows and Wildlife Conservation in Australia , CSIRO Publishing, Melbourne.
• Lindenmayer, D (2003) Wildlife on Farms – How to Conserve Native Animals, CSIRO Publishing.
• Lunney, D (ed) 2004 Conservation of Australias Forest Fauna (2^nd Edition) Royal Zoological Society of NSW.
• Jim Grant, (1997) The Nest Box Book. Gould League of Victoria Inc, Moorabbin, Australia.
  

Bibliography

1. Alan and Stacey Franks, (2003) Nest Boxes for Wildlife, A Practical Guide.  Blooming Books, Melbourne, Australia.