Large scale climate affects the timing of spring arrival but local weather determines the start of breeding in a northern little tern (sternula albifrons) population

Abstract

In migratory birds, the timing of arrival to the breeding grounds can be affected by weather on the migration routes and breeding sites at arrival. Timing of breeding can be affected by arrival dates but also by local weather conditions during the pre-breeding period. Because long-distancemigrants arrive and breed late in the season, weather is thought to be less important, butmore information is needed especially fromseabirds that are vulnerable to climatic effects. I used observation data from the Little Tern (Sternula albifrons), a long-distancemigrant,made by bird watchers during 2006-2016 to examine whether their spring arrival to the breeding grounds in Finland is associated with the large-scaleweather pattern (theNorthAtlanticOscillation;NAOindex) or localweather. In addition, I used breeding data to examine whether the start of egg laying is associated with the annual first arrival dates, local weather or the NAO index. Spring arrival was associated with large scale weather patterns rather than local weather conditions. Little Terns arrived earlier when the April-May NAO index was positive being consistent with most migratory birds. However, early arrival dates did not translate to early timing of breeding which was more determined by local temperatures before egg laying. Arriving early allows preparation for breeding, e.g., courtship feeding, and makes it possible to start breeding when conditions become suitable for egg laying, and suggests that these long-distance migrants should be able to track advancing springs and start their breeding in the most optimal time.