Large–scale geographical variation confirms that climate change causes birds to lay earlier

Advances in the phenologyof organisms are often attributed to climate change, but alternatively, may reflect a publication biastowards advances and may be caused by environmental factors unrelated to climate change. Both factors are investigated using the breeding dates of 25 long-term studied populations of Ficedula flycatchersacross Europe. Trends in spring temperature varied markedly between study sites, and across populations the advancement of laying date was stronger in areas where the spring temperatures increased more, giving support to the theory that climate change causally affects breeding date advancement.