Does interspecific competition with introduced grey squirrels affect foraging and food choice of Eurasian red squirrels?

Abstract

Grey squirrels, Sciurus carolinensis, introduced from North America, have replaced red squirrels, S.vulgaris, over much of Britain and parts of north Italy, but the reasons why are unclear. Spatial and temporal changes in the quantity and quality of their primary foods, namely tree seeds, may provide the focus for interspecific resource competition and hence go some way to explain the replacement process. To investigate whether grey squirrels have a competitive advantage over red squirrels, we used radiotelemetry and direct behavioural observations to examine the activity budget, foods, feeding behaviour and body condition of adult red squirrels in two mature, mixed-woodland sites in northern Italy, one site where there were only red squirrels, and one where both red and grey squirrels were present. The studies were carried out between July 1996 and October 1998. We found few differences in the activity and foraging patterns and food choice of red squirrels with and without grey squirrels present, although we could not eliminate possible interspecific competition effects on food choice by red squirrels in summer (June–August) and autumn–winter (September–February). Foraging time and rate of energy intake of red squirrels in the mixed-species site were lower than in the red-only site in winter (December–February). This may have resulted from interspecific competition, but a more plausible explanation is that these site differences resulted from the distribution of preferred tree seeds and home range size. Overall, our results provide little support for the food competition hypothesis. Differences in body size between sites suggest that interspecific competition occurs during the growth phase of red squirrels, when juveniles and subadults disperse and look for a place to settle.