Introduction

Humans have deliberately or accidentally introduced various plants and animals to areas outside their natural range (Long 1981; Lever 1994). European acclimatisation societies introduced many bird species from Europe into Australasia and the New World to enrich the local avifauna, but species were also introduced in order to control insect pests or as game birds. Some of these species have established populations successfully and became invasive, often with negative or even disastrous consequences for biodiversity, community structure and native species, sometimes leading to species extinction (e.g. Case 1996; Sala et al. 2000; Stein et al. 2000). Introduced animals are estimated to be responsible for about 40% of historic extinctions (Caughley and Gunn 1996). Consequently, introductions of exotic species and their impacts on native species have received much attention in recent years (see e.g. Savidge 1987; Blackburn and Duncan 2001; Duncan et al. 2003; Smith 2005). However, not all introductions of exotic species were successful; many introduced species either failed to establish or established but did not spread (e.g. Duncan et al. 2001). The majority of bird introductions into continental areas are failures. For example, in the continental United States, only 13 species of introduced birds are common, although at least 98 species have been introduced (Case 1996). On average, introductions of continental bird species are only successful about 10–30% of the time (Krebs 2001). The detailed historical records of bird introductions to New Zealand, the United States and many oceanic islands have been used extensively to test hypotheses about the factors influencing introduction success and the subsequent spread of introduced birds (New Zealand: Veltman et al. 1996; Duncan 1997; Green 1997; Sorci et al. 1998; Duncan et al. 1999; Legendre et al. 1999; Sol and Lefebvre 2000; Southeastern Florida: Rand 1980). A similarly detailed record of bird introductions to Australia exists (Long 1981; Newsome and Noble 1986); of the 52 bird species introduced to mainland Australia, only 19 (37%) species have successfully established (Long 1981; Newsome and Noble 1986) and only 5 (10%) species have become widespread and abundant in Australia (Duncan et al. 2001).