Seasonal Trends in Abundance, Diet and Breeding of Common Crossbills (<i>Loxia curvirostra</i>) in an Area of Mixed Species Conifer Plantation Following the 1990 Crossbill ‘Irruption’

Abstract

Crossbills increased in abundance on lower Deeside, north-east Scotland, from July 1990 through the autumn, remaining numerous until the following June. Flock size initially increased, but decreased towards late winter as breeding began. The timing of the population increase, and the sizes of the birds' bills suggested they were mainly immigrant Loxia curvirostra . Their diet changed seasonally in a pattern that could be explained by the seed weight, cone toughness and seedfall phenology of the predominant conifers present. No breeding was recorded in autumn and winter when the birds were mainly feeding on Sitka spruce, but commenced in early spring as they shifted to Scots pine. Breeding was late, and production poor, probably because only some of the birds had large enough bills to feed efficiently from the robust cones of Scots pine. It is argued that plantation forestry, by providing a novel range of conifer seeds, could affect selection for bill size in British crossbill populations.