Current and future impacts of nest predation and nest-site competition by invasive eastern grey squirrels Sciurus carolinensis on European birds

The eastern grey squirrel (hereafter 'grey squirrel') is considered one of the most damaging invasive alien species in Europe, with negative effects on native ecosystems. Despite it being widely perceived as a significant predator of bird eggs and chicks and as a competitor for nest sites, evaluation of the grey squirrel's impact on European bird populations has been hindered by limited empirical data. The aim was to review the incidence of grey squirrels as nest predators of and nest-site competitors with European birds, and to use this information to identify species at potential risk of negative effects from within the grey squirrel's expanding range in continental Europe. A comprehensive literature review was conducted, and the data were used alongside additional new data, to assess nest predation and competition by grey squirrels in their current European range. Bird species were grouped by nest-site type, which was used to predict the impact on similar species groups in regions of continental Europe predicted to be colonised by grey squirrels in the current century. Camera-monitoring and field evidence for 12 bird species and 12420 nests in Britain showed that grey squirrels rarely depredated eggs or chicks, affecting just 0.5% of nests. Nest-site competition was also minor, with grey squirrels occupying 0.8% of 122 small tree cavities and 14% of 57 larger cavities. At least 69 bird species in continental Europe could be exposed to potential nest predation or competition from expanding grey squirrel populations within the current century, but population-level effects currently appear to be unlikely. Current evidence shows that grey squirrels are unlikely to be significant predators of or competitors with nesting birds in their present or projected range in Europe. However, further studies of more species in different regions would be valuable, particularly in urban and suburban habitats.