The sympatric breeding of Common and Scottish Crossbills Loxia curvirostra and L. scotica and the evolution of crossbills

Within the strongly polytypic Red Crossbill L. curvirostra there are many reports of two or more 'subspecies' nesting sympatrically, without interbreeding. This 13-year study examines one such case, in Scotland, where an endemic form is resident and another was thought to occur after irruptions from its main range in continental Europe. Both forms were present in the study area every year; sympatric breeding was proved in 9 years and probably occurred in the other four. There was no suggestion of interbreeding and the Scottish form should be treated as a separate species, L. scotica , the Scottish Crossbill. This is the only endemic species of bird in Britain and one of very few European endemics. The evolution of crossbills in Europe is discussed in the light of recent palynological evidence and the taxonomic status of the Parrot Crossbill L. pytyopsittacus is re-examined.