VOL. 15 (6) JUNE 1994 Blackbirds at , N.S.W. 273

Blackbirds Reach Barrington Tops,

Blackbirds Turdus merula were successfully introduced from Europe to Melbourne before 1862, and to Adelaide in 1863. From these centres, and assisted by further releases, they have spread across most parts of Victoria, Tasmania and south-eastern South , and northwards into southern New South Wales (Blakers eta!. 1984). Records from New South Wales have been collated by Morris et a!. (1981) and McAllan & Bruce (1989). The most northerly sites appear to be Armidale (Blackbirds first recorded in 1977) and Bourke (by 1981), but colonisation has been more by 'jumps' than by steady spread and many areas further south have not yet been invaded. In the Barrington Tops region there are records of Blackbirds at Dungog (50 km south­ south-east, 70 m altitude) in 1959-60, and at (100 km west-north­ west, 500-1200 m altitude) in 1979 (Morris eta!. 1981). There are no records of Blackbirds on the Barrington Tops plateau itself before 1993.

In November 1993, I observed Blackbirds near Swamp, Barrington Tops, New South Wales (31 °58'S,151 °26'E; 515630 on N.S.W. 1:25 000 mapsheet 9134-2-S). The swamp lies in the central part of the Barrington Tops plateau, beside the Scone-Gloucester road at an altitude of c. 1450 m . I had seen none there during many visits at various seasons over the previous eight years, most recently in both January and June 1993, although there are unpublished observations of Blackbirds singing at two places nearby in January 1993: near Barrington Trail where it crosses Polblue Creek (near the south-eastern corner of the area depicted in Figure 1), and near the end of Mt Carson Trail (c. 1 km north-east of previous site), both in similar habitat to that described below (I.A.W. McAllan pers. comm.).

On 5-6 November, two males were simultaneously seen in separate territories, and at least three birds appeared to be singing in the area at dusk. Only two brief sightings of a female were made, and no nesting activity was found . The birds were occupying eucalypt woodland/open forest (Snow Gum Eucalyptus pauciflora, Black Sally E. stellulata and Mountain Gum E. dalrympleana) with a dense shrubby understorey (see below), near its margin with tussocky grassland (Snow Grass Poa sieberiana) and swamp vegetation.

More detailed observations were undertaken on 25-26 November 1993. Male Blackbirds (but no females) were seen in five separate places and were heard singing at dusk in four of these and in three other locations as well (see Figure 1) . They were shy in their behaviour, and difficult to approach. They were seen in trees (which they used as singing perches) or flying, not feeding or attending nests (none of which was found). It appears that the male Blackbirds were involved in territorial behaviour, and that other activities in which females were also involved were undertaken in part of the habitat where they could not be seen easily.

The observations suggest that at least four and perhaps up to eight male Blackbirds, with an unknown number of females, occupied territories in the area studied, which altogether amounts to about 1.5 square kilometres. However, Blackbirds were absent from the swamp and grassland which occupy nearly a third of the area, giving a density in woodland of roughly ten birds per square kilometre or 0 .1 per hectare. This value, although low by comparison with Europe and suburban Melbourne (Blakers et a!. 1984), nevertheless seems remarkable for an alien bird in a little-disturbed native woodland environment, especially at such an apparently early stage of invasion. AUSTRALIAN 274 SMITH BIRD WATCHER

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\ (\ I \ \ \\ + \ \ \ \ I \ I \ I \ 0 " ------+ Blackbird seen 0 Blackbird singing D Woodland D Swamp/grassland Approx. boundary of 0 1 00 200 300 400 500 metres surveyed area Road

Figure 1. Map of Polblue Swamp and surrounding areas showing locations of Blackbirds observed in November 1993. The climate of the area is cool and moist throughout the year, which might account for the apparently rapid population increase achieved there by Blackbirds. Furthermore, the habitat may have been rendered even more suitable for them by the ecological impact of another invading European species, the shrub Scotch Broom Cytisus scoparius. Originally planted as a garden ornamental in the north of the plateau in the 1840s, Broom spread particularly fast after removal of cattle from most of the area in the late 1960s, and now occupies over 10 000 ha. It forms an understorey in eucalypt woodlands as well as growing in pastures and around the grassy margins of swamps, and usually achieves a density sufficient to shade out most vegetation beneath it (Waterhouse 1988). In doing so, it creates large areas of nearly bare or litter-covered ground. Older Broom plants collapse under the pressure of winter snow, creating gaps in the formerly unbroken, dense canopy (Smith 1994). This appears to have provided an ideal feeding habitat for Blackbirds, which were seen several times flying into gaps in the Broom thickets. In a previous study of the birds of the same area, Bell (1990) found that at its present stage of invasion, in which the eucalypt overstorey remains intact despite the addition of a dense shrubby understorey, Broom has not markedly disadvantaged any bird species, most of which depend mainly upon resources provided by the tree component of the vegetation. However, several bird species characteristic of ground and shrub layers (Brown Thornhill Acanthiza pusilla, Eastern Yellow Robin Eopsaltria australis, Olive Whistler Pachycephala olivacea, Silvereye Zosterops latera/is, Spotted Pardalote Pardalotus punctatus, Yell ow-faced Honey eater Lichenostomus chrysops, VOL. 15 (6) JUNE 1994 Blackbirds at Barrington Tops, N.S.W. 275 and in particular Eastern Whipbird Psophodes olivaceus and White-browed Scrubwren Sericornis frontalis) were significantly commoner in Broom-infested vegetation. Of these, only the Eastern Whipbird seems likely to compete for food with Blackbirds, both feeding on invertebrates under leaf litter on ground beneath dense shrub canopies. White's Thrush Zoothera dauma, closely related to the Blackbird and another possible competitor, is less common in the area. Bell (1990) recorded densities of c. 0.34 and 0.06 per hectare for Eastern Whipbird and White's Thrush respectively in Broom­ infested sites in the area. It might be expected that in their newly found, nearly vacant ecological niche at Barrington Tops, Blackbirds in the short term will increase further in numbers and local range, especially in areas over-run by Broom. Longer-term changes in the environment, and their effects upon the avifauna, are difficult to foresee but will be of great interest. They may involve Broom-induced deterioration of the eucalypt overstorey or, conversely, a thinning of the Broom thickets as a result of a biological control program currently under way . I thank Stephen Debus, Hugh Ford and I.A.W. McAllan for useful criticisms of earlier versions of this paper.

References Bell, S.A.J. (1990), Effects of the Weed Scotch Broom on Bird Communities in Open Forests on Barrington Tops, honours thesis, Dept Geography, University of Newcastle. Blakers, M., Davies, S.J.J.F., & Reilly, P.N. (1984), The Atlas of Australian Birds, Royal Australasian Ornithologists Union, Melbourne. McAllan, I.A.W. & Bruce, M.D. (1989), The Birds of New South Wales: a Working List, Biocon Research Group, Sydney. Morris, A.K., McGill, A.R. & Holmes, G. (1981), Handlist of Birds in New South Wales , N.S.W. Field Ornithologists Club, Sydney. Smith, J.M.B. (1994), 'The changing ecological impact of Broom (Cytisus scoparius) at Barrington Tops' , Plant Protection Quarterly 9(1), in press. Waterhouse, B.M. (1988), 'Broom (Cytisus scoparius) at Barrington Tops, New South Wales ', Aust. Geographical Studies 26, 239-248. By J.M.B. SMITH, Department of Geography and Planning, University of New England, Armidale, N.S. W. 2351 Received 21 December 1993 •