Ben Swallow

University of St Andrews

Title: Hierarchical GAMs for studying the irruptive migration of crossbill species in northern Europe

Abstract: Irruptions by seed-eating birds are assumed to be driven by the production of seeds and fruits, whose crops are highly variable between years. Using data from Sweden, Finland and UK, we tested a variety of assumptions about synchrony in coniferous seed crops and whether irruptions of crossbills Loxia sp. were correlated with seed production further afield. In a second set of analyses, we developed hierarchical generalised additive models to study when irruptions into northern Europe took place. The models indicate that the incidental co-occurrence of low seed production of Norway spruce and Scots pine in a given year, after a year of high seed production, may result in an irruption. The seed production of Norway spruce and Scots pine in Sweden was correlated with production by the same species in Finland, indicating widespread synchrony of cropping across northern Europe.