Cone sizes of Scots pines Pinus sylvestris in the Highlands of Scotland - implications for pine-eating crossbills Loxia spp. in winter

摘要

Crossbills Loxia spp. and squirrels, Sciuridae, are two groups of vertebrates that commonly feed on seeds secured between the closed scales of conifer cones when the cones are still attached to the trees (Smith and Balda, 1979). Within Scotland, there are three species of crossbills: common crossbills (Loxia curvirostra), parrot crossbills (L. pytyopsittacus) and Scottish crossbills (L. scotica), though the parrot crossbill is a rare breeder (Gibbons et al., 1993). The latter two feed primarily on Scots pines Pinus sylvestris (Witherby et al., 1943) and the Scottish crossbill is accepted as being Britain's only endemic bird (BOURC, 1980). By being restricted in range (north-east Highlands of Scotland) and habitat (principally old pine forests), and in the absence of good data on its status because it is difficult to identify in the field, the Scottish crossbill has been classed as globally endangered (Tucker and Heath, 1994). The red squirrel Sciurus vulgaris is widespread in Scotland and feeds largely on Scots pines (Tittensor, 1975; Moller, 1986; Gurnell, 1987). It too is of conservation concern as it has contracted in range and has been replaced in many places by the introduced grey squirrel Sciurus carolinensis (Gumell and Pepper, 1993; Anon, 1995).