Predation of woodland bird nests by tree squirrels in Britain, Central Europe and North America
We review recent nest camera studies of the fate of woodland bird nests in Europe and North America and present summary data of predation events by tree squirrels. We also review the findings of several recent correlative studies that used long term datasets on bird populations and squirrels from the United Kingdom (UK). Finally, we present breeding bird data from a UK squirrel control experiment. Eighteen camera studies from North America indicated that the American pine squirrel is a common predator of bird nests there. Grey squirrels were occasional predators in three of these studies. Three single species camera studies in the range of grey squirrels in Britain found no predation by them while two studies in the Czech Republic both found varying levels of predation of various bird species by Eurasian red squirrels. These camera studies do not quantify population level effects. The UK correlative studies address specific questions about the impact of changes in grey squirrel numbers on adult bird populations on a variety of bird species, two of which showed reduced population size in the presence of greys in two separate analyses. Data derived so far from a grey squirrel control experiment are inconclusive but do not rule out the possibility that squirrel reduction can sometimes lead to improved breeding success in birds. We look at learning and behavioural mechanisms in squirrels that might cause highly variable levels of predation between individuals or groups. We suggest nest monitoring studies might miss grey squirrel predation because they may predate canopy nests preferentially.