In the absence of any control, grey squirrel populations vary considerably between years, due to fluctuations in natural food availability. Gurnell (1996) recorded a 6-fold variation in summer population size from 2.8 to 17.9 squirrels per hectare for one site over a 13-year period. This is one reason why it is very difficult to predict either risk of damage or control effort required in a given year. Densities in mature large seeded broadleaved woodland are often around 7-9 per ha, but damage appears to be triggered by densities of 5 or more per ha.
We are developing a method to predict likely risk of damage in early January to enable woodland owners to be more effective in targeting control from mid March onwards. Stands of vulnerable age tree species close to good squirrel habitat are most at risk of damage when squirrels survive well over winter and breed in early spring. This is most likely to occur if there is plenty of natural (or other eg. pheasant feed) available over winter.
To test this assumption we are attempting to trap squirrels in 14 sites in early January and assess their breeding condition. At the same time we are assessing natural food availability and level of squirrel presence in these sites. If there is no natural food we expect to be able to catch squirrels and check whether they are breeding.
In October we record levels of damage to the adjacent vulnerable stands of trees.
Data collection is continuing due to the high levels of variation in squirrel densities and damage across sites that we have recorded so far.
