Chris Johnston recently posted an interesting note regarding some mysterious scarring on a eucalypt. As Chris suspected the culprits are in fact Galahs Eolophus roseicapillus. Hopefully what follows will help clear up the mystery. Yesterday I came across a pair of Galahs on Mia Mia Track at what appeared to be a new nesting hollow. One of the birds was carrying a spray of eucalypt leaves.
Galahs at nesting hollow - one bird with spray of gum leaves, Mia Mia Track, 12th September 2011
The behaviour of this familiar and beautiful species has been well-studied. During the breeding season sprays of eucalypt leaves are cut by the male then brought to the nest site and shaken as part of an elaborate courtship ritual. The spray is then usually taken by the female while the male cuts another spray from nearby and rejoins the female, whereupon both engage in some joint leaf shaking. The leaves are then generally poked inside the hollow to line the nest, with many bunches dropped in the process! This particular behaviour is most frequent about one week before egg-laying starts. I then watched on as the pair preened each other around the head and throat, a behaviour known as allopreening.
The pair allopreening at the nest hollow.
About a metre below the nest site was a large scar, almost encircling the trunk of the tree. These scars are formed often on Galah nest-trees as a result of the birds systematically stripping the bark to expose the cambium, which then dries to form a scar. Sometimes the bark and cambium is removed to expose the underlying xylem tissue, sometimes killing the tree. Pairs usually enlarge the scar each year by chewing at the edges.
Galah at hollow with scar below, Mia Mia Track, 13th September 2011.
Why do they do this? There have been a number of theories put forward ranging from; deterring predators such as goannas, bill cleaning and sharpening, territorial marking and even a suggestion that the birds do this to ensure a future supply of nesting hollows. The following link to an extract from Ian Rowley’s ‘Behavioural Ecology of the Galah, Eolophus roseicapillus, in the Wheatbelt of Western Australia’ (Surrey Beatty, 1990) makes interesting reading.
Natural Newstead 
Wonderful information – thanks Geoff!