Monitoring red squirrelsSciurus vulgarisand grey squirrelsSciurus carolinensisin Britain

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Abstract
1. In Britain, five indirect methods have been used to survey for the presence of redSciurus vulgarisand grey squirrelsS. carolinensisand to monitor population trends through time. These are: visual counts along line transects, the use of hair tubes, counts of dreys, cone line feeding transects in conifer forests and feeding signs on whole maize bait at feeding stations.2. Drey counts and cone feeding transects cannot differentiate between red and grey squirrels and are not suitable for areas where both species may be present. Hair tubes require expertise and facilities to identify hairs to species, and the relationship between tube visits and animal density in different habitats is only known for red squirrels in Sitka sprucePicea sitchensisdominated plantations. Visual counts along line transects are the simplest method to do and require little equipment. However, in general, few squirrels are seen in a single count which leads to difficulties in estimating densities.3. In this paper we present data from several studies carried out in Britain to illustrate the type of data that may be collected, and to evaluate their accuracy and precision with which they could detect population change. Hair tube lines and drey counts required the fewest number of samples to detect population change. When effort was also considered, visual transects and drey counts were more economical. However, none of the monitoring methods discussed is very precise and it may be best to monitor squirrels only in terms of relative changes in numbers or indices of numbers through time.4. Our results for red and grey squirrels also illustrated that the number of samples required to detect population change is likely to differ according to the size of the survey area. Monitoring programmes should therefore be carefully planned in relation to spatial scale in order to be able to detect real population changes. Species‐specific, quantitative thresholds to determine the significance of detected declines may need to be established for threatened UK mammals and assessed in relation to what would constitute a significant decline and whether it requires management intervention.

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