Evaluating the population-level impact of an invasive species, Ring-necked Parakeet Psittacula krameri, on native avifauna STUART E. NEWSON, 1 * ALISON JOHNSTON, 1 DAVE PARROTT 2 & DAVID I. LEECH 1 1 British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK 2 Wildlife Management Programme, The Food and Environment Research Agency, Sand Hutton, Yorkshire, YO41 1LZ, UK The introduction of exotic species to ecosystems can have severe consequences for populations of native organisms, but logistical limitations and shortage of historical data often hinder attempts to quantify the ecological implications of such relationships. The establishment and rapid expansion of Ring-necked Parakeets Psittacula krameri in England therefore presents a rare opportunity to apply novel analytical methods to existing extensive national bird monitoring data from the UK Breeding Bird Survey for an invasive species. A previous study from Belgium suggests that Ring-necked Parakeets have the potential to reduce the abundance of Eurasian Nuthatch Sitta europaea through competition for nesting cavities. Our analysis provides no evidence for a significant impact through competition on Nuthatch populations or those of any other cavity-nest- ing species within the Parakeet’s current range in the UK. However, we cannot exclude the possibility that competitive exclusion could be occurring at a minority of sites at which availability of nest cavities is limiting. This may yet have significant implications for future conflict if Parakeets continue to increase in numbers and range. Keywords: cavity-nesting bird, competition, invasive species, rates of population change, Ring-necked Parakeet. When novel organisms are introduced to an eco- system, consequences for native species can be severe (Gurevitch & Padilla 2004, Clavero et al. 2009). Impacts may be direct, through predation, competition or hybridization, or indirect, via the introduction of new parasites and pathogens. Once an exotic species becomes established, it is neces- sary that any impacts are quantified as quickly as possible in order to implement effective mitiga- tion. However, efforts to quantify impacts may be compromised by a lack of data on native species prior to the establishment of the invasive species and by the logistical restrictions of carrying out monitoring at large spatial scales (Wittenberg & Cock 2001). In North America, for example, the non-native European Starling Sturnus vulgaris and House Sparrow Passer domesticus may compete for nesting sites with native cavity-nesting birds (Ingold 1998), but because the populations of these species became established in the 19th century, the patterns and processes occurring at their initial colonization are poorly understood and the influence of these species on native species remains a subject of debate (Koenig 2003). Studies of the relationship between invasive species and indigenous birds in the UK are facilitated by the existence of long-term monitoring programmes such as the British Trust for Ornithology (BTO) / RSPB / Joint Nature Conservation Commit- tee (JNCC) Breeding Bird Survey (BBS), which records changes in avian abundance at a national scale (e.g. Newson et al. 2008). The focus of this study is to examine the evi- dence for a population-level impact of Ring-necked Parakeets Psittacula krameri on native cavity- nesting bird species in the UK. The Ring-necked Parakeet is the most widely introduced parrot in the world, with breeding populations established in 35 countries in five continents (Butler 2003). Ring-necked Parakeets became established in the wild in the UK in the 1970s after captive birds *Corresponding author. Email: stuart.newson@bto.org ª 2011 British Trust for Ornithology Journal compilation ª 2011 British Ornithologists’ Union Ibis (2011), 153, 509–516