Experimental evidence for nest-site competition between invasive ring-necked parakeets (Psittacula krameri) and native nuthatches (Sitta europaea)

Ring-necked parakeets (Psittacula krameri) originate from Africa and Asia, but have been widely introduced and are thought to compete for nest-sites with native hole-nesting birds, raising concerns for the loss of native avifauna. In this study, we adopt a replicated before–after-control experimental design to test for competition between parakeets and native cavity-nesters. In experimental plots in Brussels, Belgium we blocked parakeet nesting cavities and conducted surveys to evaluate availability and use of cavities by parakeets and native hole-nesters. Numbers of breeding pairs, as well as cavity quality index – based on cavity occupancy rates – were evaluated before and after blocking. After blocking, numbers of native nuthatches (Sitta europaea) declined significantly, largely due to nest take-overs by parakeets. Nuthatches and parakeets strongly overlap in preferred nest-sites and this overlap explains the competition that leads to a reduction in nuthatch numbers when suitable cavities become scarce. Average quality of parakeet nests was lower after blocking, indicating that parakeets will utilize alternative cavities when preferred sites are not available, bringing them into greater conflict with other cavity nesters. Although some nuthatches remain in areas dominated by ring-necked parakeets, our results raise conservation concerns for the vulnerability of native cavity-nesting birds to the expansion of invasive parakeet populations.