ON THE SELECTION OF TREE SPECIES BY ACANTHIZIDAE IN OPEN-FOREST NEAR SYDNEY, NEW-SOUTH-WALES

Selection between tree species as foraging substrates by four species of Acanthizidae was studied in an open eucalypt forest near Sydney, New South Wales. Weebills Smicrornis brevirostris, Striated and Little Thornbills Acanthiza lineata and A. nana foraged more often than expected on the foliage of Narrow-leaved Ironbark Eucalyptus crebra than on that of Grey Box E. moluccana. By contrast Gerygone olivacea used the two trees in about the same proportion as the amount of foliage of each species. Weebills and Striated Thornbills selected Narrow-leaved Ironbark foliage as a foraging substrate more often than Little Thornbills in winter but not in spring. Little Thornbills foraged on Narrow-leaved Ironbark foliage more often in spring than in winter. The selection of eucalypt foliage as a foraging substrate is correlated with the abundances and kinds of invertebrates found on the two tree species. Invertebrates were generally more abundant on the foliage of Narrow-leaved Iron-bark than on that of Grey Box in both winter and spring. Lerp-forming psyllid insects, however, were more abundant per leaf on Grey Box than Narrow-leaved Ironbark in winter, but not in spring, Grey Box leaves are larger than those of Narrow-leaved Ironbark and when the abundances of psyllids were corrected for leaf area (i.e. as numbers/cm(2)), the difference between Grey Box and Narrow-leaved Ironbark in winter was not significant. However, in spring the numbers of psyllids/cm(2) on Narrow-leaved Ironbark foliage was greater than on Grey Box foliage. The differences in invertebrate abundances account for some of the differences in the use of the two eucalypts as foraging substrates, but variability in invertebrate abundances between leaves and individual trees, as well as differences in foliage morphology, may also have been important.