Gang-gang hollows, hot property in South-West Sydney? Artificial Hollow deployments in Urban Remnant Bushland

University of Sydney Honours student Liam Doherty has just completed his thesis looking at the potential for different types of artificial hollows (aka not the traditional timber nest boxes) to provide habitat for Gang-gang Cockatoos. This research was funded by the NSW Twitchathon and done in collaboration with BirdLife Australia and Campbelltown City Council. Liam writes:

The loss of mature trees and the tree hollows that form in them has become a major concern in urban areas, especially in remnant bushland. This critical habitat infrastructure provides shelter and breeding areas for over 300 different vertebrate species across Australia. One of the ways to combat hollow loss, especially in urban ecosystems, is to supplement habitat with artificial hollows. These hollows aim to replicate naturally occurring tree hollows, and supplement the habitat until suitable natural hollows can form.

Over the past nine months, I have been intensively monitoring the usage of different types of artificial hollows for my honours project at the University of Sydney in partnership with BirdLife Australia and Campbelltown City Council. We aimed to monitor three different artificial hollow types and compare them to natural hollows in the area. We specifically looked at which species visited, entered and occupied the hollows, as well as looked at the internal temperature and humidity of all these hollow types across the duration of the summer breeding season.

As well as taking a broad look at everything that uses the hollows, we wanted to specifically target a threatened species to see if targeted deployments of artificial hollows can help as a conservation initiative. Gang-Gang Cockatoos (Callocephalon fimbriatum) are a species that heavily rely on hollows, and are a species that have lost a large amount of habitat due to the 2019-2020 bushfire season. They also struggle to compete for suitable hollows in urban and semi-urban areas with urban opportunist species such as Sulphur-crested Cockatoos and Rainbow Lorikeets. Having been observed in and around Campbelltown in the past, we thought they would be a suitable candidate to target with a hollow deployment project.

We deployed 19 wildlife camera traps on 15 artificial hollows and 4 natural hollows at three different sites around Campbelltown, in remnant bushland areas where Gang-Gang Cockatoos have previously been observed. The three different artificial hollow types we analysed were supplementary hollows installed in trees using chains or tension cables, chainsaw hollows (also called carved hollows or hollow hogs) that were carved into dead or partially dead sections of trees, and a modern, modular, style of nest produced by Habitat Innovation and Management using a combination of a plastic outer and wooden inside.

We also deployed dataloggers to monitor both internal and external temperature and humidity, as well as performed bi-weekly bird assemblage surveys to record what species are found at each site across the duration of the breeding season.

Here were the man findings:

1. There was no distinct pattern of hollow use between the different types of hollows or the species that used them, everything used everything! All hollows were visited by a range of different species, with Rainbow Lorikeets, Eastern Rosellas and Galahs inspecting and entering hollows the most. This was consistent across all three of our sites. We also had three species use artificial hollows as nests, an Australian Wood Duck, an Eastern Rosella and a Sulphur Crested Cockatoo.
2. Temperature and humidity across all hollow types was very similar. When averaged out across a 24-hour period, all artificial hollows closely followed the temperature and humidity fluctuations observed in natural hollows.
3. Bird assemblage surveys showed a difference between sites, but there was no change in assemblages over the breeding season. This means that the same species were consistently present at each site. Yet this difference in site assemblages was not reflected in the visitation to the hollows.
4. Gang-Gang Cockatoos were observed using hollows, but we did not record any nesting in any of the hollow types by the target species. This shows that our target species showed an interest in the hollow deployments, and despite no nesting recorded it gives a promising base for future deployments of hollows targeting this species. Gang-Gang Cockatoos were observed at two of the three sites during bird assemblage surveys, but were observed on camera inspecting hollows at the other. This shows that species may still be using areas despite us not observing them when on the ground.

What does this mean for future artificial hollow deployments, and how should they be managed in order to be successful?

Artificial hollow deployments have been shown to contribute greatly to biodiversity conservation, but are not always successful for the conservation of specific species. This makes deployment and management of artificial hollows quite tricky. Especially in urban or remnant bushland areas, common species such as Rainbow Lorikeets and Sulphur-crested Cockatoos almost always occupy hollows before other less common species. Often, factors that are favoured by threatened species overlap with these opportunist species, making it hard to successfully target less common species.

What this study did show is that an intensive management approach highlights the importance of hollows in an ecosystem more than traditional nest box deployments. The success of artificial hollow or nest box projects is often judged by how many boxes are deployed, or by how many nest boxes become nests. This is usually done by checking boxes manually or with a pole camera, and often very infrequently. If this approach was performed on this study, it would have shown that these hollows were not successful due to a low nesting uptake. Gang-Gangs would also not have been observed utilising the hollows as they were on our cameras. By changing what we consider to be a successful deployment, we showed that these artificial hollows really do help areas lacking in hollows.

We recommend nest box or artificial hollow deployments to consider an intensive management approach, and reconsider how we judge what a successful deployment is. This may often require long-term deployment of these hollows, which can require maintenance, but will likely provide better outcomes for threatened species. We also stress that despite not observing a target species nesting in the hollows, they may still contribute to helping the ecosystem.