The effects of food availability and winter weather on the dynamics of a grey squirrel population in southern England.
The population ecology of grey squirrels (Sciurus carolinensis) in a 9-ha oak (Quercus robur) woodland in Alice Holt Forest (Hampshire/Surrey) was studied in 1976-87 using live trapping techniques, in winter, spring and summer. The availability of tree seeds during autumn, and the severity of winter weather, were also recorded to determine their effects on squirrel population dynamics. Capture probabilities of squirrels in winter (and to a lesser extent in spring) were inversely related to food availability, and data from these seasons were consequently not considered reliable; most of the analyses reported here were done with summer populations. Long-term summer densities were high (8.8/ha) and similar over 10 of the 12 yr analysed. In good seed years breeding started in December, but in poor years it was deferred until spring. There was no or little spring breeding in 5 yr with poor food supply, and female breeding success was positively associated with food availability. Persistence from summer to winter was also associated with food availability, while persistence from summer to spring and the following summer were not. Based on the data analysis, a general linear model was derived for the relations between numbers of squirrels in summer populations and sex, food availability and severity of winter weather; this accounted for 77% of the variation found in squirrel numbers between years. Tree seed availability was the most important factor limiting squirrel densities, and this interacted both positively and negatively with winter weather severity.