BRANTA — Ben Swallow
Coincidental bird irruptions
Institution: University of St Andrews, UK
Supervisors: S Buckland
Details: MSc 2011 (Completed)
Address: CREEM, The Observatory, Buchanan Gardens, St Andrews, Fife, KY16 9LZ (Oct 2012) Email
Subject Keywords: crossbills, migration, irruptive migration, conifers
Species Keywords: Common Crossbill, Parrot Crossbill, Two-barred Crossbill
Abstract
We analysed data concerning Norway spruce Picea abies and Scots pine Pinus sylvestriscone indices from Sweden during the years 1909-1953 in relation to the irruptions of common Loxia curvirostra, parrot L. pytyopsittacus and two-barred L. leucopteracrossbills into southwest Europe.
- Norway spruce was synchronous over the whole of Sweden for the period studied, with Scots pine synchronous over all but the greatest distances.
- Common and two-barred crossbill irruptions were correlated with poor Scots pine crops, whilst parrot crossbills were more likely to move when Norway spruce crops failed.
- There was some evidence to suggest for common and parrot crossbills that a combined failure of the two cone species increased the likelihood of irruptive behaviour.
- There was no correlation between a good cone crop one year and an irruption in the following year, suggesting that the population density of crossbills in Sweden had little or no effect on the likelihood of irruptions.
- The arrival dates of crossbills have been linked to some extent with distance travelled (Marquiss et al. 2008). Arrivals in September or later could not be explained solely by distance travelled, but would be better described in relation to a combination of distance travelled and timing of the moult. We propose that early irruptions consist mainly of juvenile birds, with some un-moulted adults, whilst later arrivals are more consistently adults that have postponed movements in order to complete their moult.