A new survey of tree hollows and their occupants points to a competitive housing market for urban wildlife

Video-footage captured by automated cameras positioned high in the canopy has revealed that many different species of bird, mammal, reptile and insect sometimes compete for the same tree hollows. By identifying the hollow “owners”, we discovered that different species have different housing preferences. Conserving an abundant and diverse supply of tree hollows is an important step towards maintaining urban biodiversity.

More than 360 species of mammals and birds worldwide make their homes in tree hollows - including 90% of Australia’s parrots. Urbanisation reduces hollow-nesting opportunities both through the removal of trees, and through the removal of dead and decaying branches (future hollows) for reasons of public safety. Hollow shortages are likely to change the composition of urban fauna communities and the nature of the change may depend on whether different species prefer different types of hollow.

As part of his Ph.D. project in urban ecology, Dr Adrian Davis learned single-rope climbing so he could take to the trees and survey the arboreal real-estate market. Supervised by Dr Charlotte Taylor (University of Sydney), and myself, Adrian counted, measured and described the hollows in 264 sites across Sydney - located in parks, streets, golf courses and remnant vegetation.

To determine whether particular species of parrot were associated with specific hollow characteristics, motion-activated video-cameras were installed to identify hollow users.

The number of both hollows and hollow-bearing trees varied significantly among habitats, with all urban habitats having significantly fewer hollows than continuous forest. Thirty-one animal species were recorded in over 1,500 independent hollow visits, with two species dominant: the Rainbow Lorikeet (Trichoglossus haematodus) and Sulphur-crested Cockatoo (Cacatua galerita). These species showed preferences for particular types and orientation of hollows, as well as particular tree species. For example, Sulphur-crested Cockatoos prefer more vertically-orientated hollows, at greater heights, located in Angophora costata trees.

Knowledge of the distribution of hollow-bearing trees throughout landscapes, as well as the characteristics of hollows that are associated with particular species, is crucial for conservation of populations of hollow-dependent fauna in urban areas.

Dr Richard Major
Principal Research Scientist

More information:

Davis A., Major R.E. and Taylor C.E. 2014. Distribution of tree-hollows and hollow preferences by parrots in an urban landscape. Emu 114: early on line

Davis A., Major R.E. and Taylor C.E. 2013. Housing shortages in urban regions: aggressive interactions at tree hollows in forest remnants. PLoS ONE 8: 1-9.