The biometrics of invading common crossbills loxia curvirostra in britain during 1990-1991

Abstract

The large invasion of Common Crossbills into Britain in 1990 from continental Europe gave the opportunity for the collection of measurements from many birds. There was no tendency for birds arriving early and late at northern migration stations to have different bill depth distributions. However, once settled in different parts of Scotland and England, there were significant differences in bill measurements between locations. Although there was a tendency for large-billed’ birds to be caught in areas dominated by pines and for ‘small-billed’ birds to be found in areas of spruce, the pattern could also be interpreted as due to chance differences in measuring technique by the different observers. Thus, we cannot be sure that there was either segregation of Common Crossbills according to bill size, or that Scottish Crossbills were occurring outside their presumed range. Also, given the possibility of shrinkage in the bills of museum specimens, there is uncertainty in using measurements of museum skins to compare with measurements of live birds. The masses of the birds on arrival at the northern migration stations were lower than those caught later in woodland in mainland Britain. No mid-winter fattening was recorded but masses increased during summer 1991 prior to the departure of many birds. © 1996 Taylor and Francis Group, LLC.