Observing hollows until animals emerge or enter can be a resource-intensive exercise. Alternatively, some hollow-nesting species leave clear evidence that the hollow, or tree, has been occupied. Many cockatoos and parrots chew the entrance of the hollow or other parts of the nest tree, behaviour which is important for maintaining the health and condition of the beak (Rowley 1990). However, birds often chew vigorously around the nest hollow, which suggests that chewing has other functions. Species that mark the nest tree permanently are among those that show high fidelity to a nesting hole. The Galah, for example, defends its nest hollow even outside the breeding season and will frequently re-use the same nest hole in successive breeding seasons (Rowley 1990). The Gang Gang Cockatoo also marks its nest tree with visible scars and shows fidelity to the same hollow over successive breeding seasons (P. Gibbons, personal observation). Marking the area around the hollow may therefore be a strategy for territorial defence. Signs of chewing around the entrance to a hollow also may be an attempt by animals to prevent hollows from being compartmentalised or occluded. This has been observed in hollow-bearing trees subject to a prolonged study in Europe (Wesolowski 1995).
Some mammals also excavate the entrance to their hollow. Trees used by Leadbeater's Possum often have distinctive cavities shaped like a 'key-hole', which are a useful way to determine if the species has occurred in (or still uses) a given patch of forest (Lindenmayer and Meggs 1996) (Figure 6.6). Other species of arboreal marsupials are known to modify the entrance hole of occupied cavities. For example, Menkhorst (1984b) observed marks around the entrances of nest boxes used by the Mountain Brushtail Possum. Similarly, the Common Brushtail Possum chews the entrance and interior of hollows.
Wear from repeated entries and exits through hollow entrances can be observed on trees. The extent of wear is partly dependent on bark type. Fibrous-barked tree species exhibit highly visible patterns of wear (Figure 6.7). Primary dens used by the Greater Glider and Yellow-bellied Glider can also be located in forests dominated by fibrous-barked species from accumulated wear on 'landing' trees adjacent to the hollow (Fleay 1947, Triggs 1998). These are trees where animals complete inter-tree gliding movements. Smooth-barked or gum-barked trees also reveal scratch marks. Numerous scratch marks on the trunk of a hollow-bearing tree can be indicative of occupancy by possums and gliders. Inions etal. (1989) identified den trees used by the Western Ringtail Possum and Common Brushtail Possum from the 'scratch-track' down the bark of the trunk. Ruibal (1998) found an association between the occurrence of large hollows and scratch marks on the trunks of Spotted Gum. Watkin Tench (1791 cited in Flannery 1994b) noted that Australian Aborigines examined trees 'to see if they could discover on the bark any marks of the claws of squirrels and opossums, which they said would show whether any of those animals were hidden among the leaves and branches'. Birds may also leave obvious scratches around the entrance of hollows on smooth-barked trees.
Although signs of chewing and scratch marks are evidence of occupancy, these signs should not be the sole basis for identifying trees, or hollows, occupied by fauna. Of 201 occupied hollows observed in tall forests, only 26% had obvious wear around their entrance (Gibbons unpublished data). Selecting trees only on the basis of evidence of visible wear will be biased
TREE HOUOWS AND WILDLIFE CONSERVATION IN AUSTRALIA
towards certain species and bark types, and may occur at hollows that are regularly inspected by animals, but not necessarily utilised.
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