Audaciously early in the morning on the 4th and 5th of October, me and a team of students, joined Dr Phillip Gibbons to help out with the annual bird surveys. The bird survey’s, are part of an ongoing 6-year study into the biodiversity value of old trees for various bird species in the ACT.
Context of Study:
With the rapid expansion of Canberra, multiple new suburbs and developments are occurring at an ever-increasing rate. As is often the case, when developers get access to a site, the first move is to rid the land of all the trees and start building with a clean slate. Under current management practises, mature tree decline of 87% is predicted in urban greenspaces (Le Roux et al, 2014). Removing trees, especially old mature ones, is destroying a significant proportion of habitat for various bird species in the ACT. Although developers are required to secure, protect and manage an offset area to balance their disturbance (Environment Protection and Biodiversity Conservation Act 1999) preliminary results from this study indicate there is inherent biodiversity value in mature trees relative to younger ones.
Sites and Locations:
On morning of the 4th we ventured out, despite the early morning cold, to a private farm in Kinlyside and observed 4 trees on this property. After we had conducted the bird surveys for these trees we moved onto an urban tree situated on a pathway between 2 residential property’s in Nicholls as well as another tree next to a roundabout in Ngunnawal. The next morning, we went east to Majura and observed 2 trees on the slopes of a private farm and 3 trees on the flats. The figure below has red circles around the sites and trees we observed whilst volunteering.
Methodology – What we Did:
In order to collect useful data that describes the relative importance of older trees in the landscape, at each site we observed bird interactions in young, medium and mature trees. Surveys were conducted in the springtime to correspond with peak bird breeding season and hence bird abundance. Moreover, at each site we would locate trees which were involved in the study (tagged with a nail, however Phil had a surprisingly good memory about the locations of these trees) and recorded various bits of information about the birds which arrived at these trees over a 20-minute period. For example, when a bird landed on a tree we would try and identify its species, the direction in which it came from as well as its departing direction, the bird’s behaviours on the tree (foraging, nesting, or attacking), resources used (live branch, dead branch, angle of branch used, hollows, foliage and seeds), aggressive species interactions as well as the duration of stay. Although it sounds easy enough, this job was actually really difficult. At a mature tree wedged between 2 residential properties in Nicholls (pictured below), we watched an Australian Magpie (Cracticus tibicen) go to town on numerous Indian Mynah birds (Acridotheres trisits) and 2 Wood Ducks (Chenonetta jubata) as it protected its nest. It was almost impossible to record all the interactions that occurred over this period. It was intense!
Species of Birds we Observed:
Over the course of our field work we observed numerous bird species in the different trees. Below is a list of the individual species we saw at the different sites.
- Eastern Rosella
- White-plumed Honeyeater
- Common (Indian) Mynah
- Common (European) Starling
- Noisy Miner
- Crimson Rosella
- Striated Pardalote
- Spotted Pardalote
- Australian Magpie
- Noisy Friarbird
- Pied Currawong
- Crested Pigeon
- Wood (Maned) Duck
Preliminary Results and Apparent Trends:
Although the bird survey study is ongoing, the trend in the data, and even on our short visit, is that bird abundance and species richness if far greater on the older trees relative to the younger ones. According to results already published, around 29% of bird species have been exclusively recorded at large trees only (Le Roux et al, 2015). Hollow-bearing trees seemed to attract the largest number of birds; as we observed species such as the Crimson Rosella (Platycercus elegans) and Striated Pardalote (Pardalotus striatus) which would use such hollows for habitat and nesting. Another trend we noticed is that old trees in paddocks with no surrounding trees seemed to do a lot better than those in density populated regions. It is common belief among land holders that trees in isolation are of little value, however results of the study are indicating otherwise. Branch angle was also a significant variable in the bird surveys. Old trees tend to have a greater number of lower angular tree branches (0 to 5 degrees) relative to younger trees which have steep branch angles (45 degrees and greater). In the field, it was obvious to see that birds could perch much more easily on a branch of 0 degrees compared to one at 45 degrees. Furthermore, tree sites that had a large collection of dead wood at the base of the tree had a positive relationship with biodiversity richness. Phil pointed out, at a Blakely’s Red Gum Tree on the Majura Site (pictured below), that the dead debris on the ground provided a good habitat for many insects which as a flow on effect provided food resources for birdlife. This is another reason why species richness was much higher at the older tree sites.
What I took away from this experience is that old trees are vitally important for biodiversity. Although it makes a lot of sense, observing this in the field really clarified my understanding. Old trees may look less ascetically pleasing then younger trees however I and a lot of other people value beautiful bird species, such as the Eastern Rosella. Therefore, to stop increasing the list of threatened and endangered species, we need to plan and consider development that keeps mature trees in the both the urban and regional landscape.
Le Roux, D, Ikin, K, Lindenmayer, D, Manning, A and Gibbons, P, 2014. The Future of Large Trees in Urban Landscapes, PloS ONE 9(6): e99403.
Le Roux, D, Ikin, K, Lindenmayer, D, Manning, A and Gibbons, P, 2015. Single Large of Several Small? Applying Biogeographic Principles to Tree-Level Conservation and Biodiversity Offsets, Biological Conservation 191: 558-5566.
Author: Alexander Clark (u5584065)