European amphibians and birds have been breeding consistently earlier over the past two to three decades¹,². These changes have been attributed to the observed trends in increasing average spring temperatures in Europe³ producing earlier growing seasons⁴ and increased forage availability. Here we show that variations in breeding of European amphibians and birds are influenced by variations in a natural, large-scale atmospheric phenomenon, the North Atlantic Oscillation (NAO). Our results support the proximate cause (that is, increase in spring temperatures) of the altered breeding phenology as suggested previously¹,², but by extending previous analyses as well as integrating data from other bird species, they also suggest that organisms with complex life histories respond to year-to-year variations in their abiotic environment.

1. Division of Zoology, Department of Biology, University of Oslo, P.O. Box 1050 Blindern, N-0316M Oslo, Norway
   e-mail: Email: n.c.stenseth@bio.uio.no