This work was supported by funds from: European Research Council (ERC-2013-StG-337365-SHE to A.Ch., ERC-2013 -AdG-339092-E-Response to M.E.V., ERC-2014-StG-639192-ALH to T.R.), Ministry of Economy and Competitivity, Swedish Research Council (621-2014-5222 to B.H.), Spanish Research Council (CGL-2016-79568-C3-3-P to J.C.S.), US National Science Foundation (DEB-1242510 to F.J., DEB-0089473 to F.S.D.), the Academy of Finland (project 265859 to T.E.) and the US Department of Energy (Award Number DE-FC09-07SR22506 to the University of Georgia Research Foundation to D.S.).

Biological responses to climate change have been widely documented across taxa and regions, but it remains unclear whether species are maintaining a good match between phenotype and environment, i.e. whether observed trait changes are adaptive. Here we reviewed 10,090 abstracts and extracted data from 71 studies reported in 58 relevant publications, to assess quantitatively whether phenotypic trait changes associated with climate change are adaptive in animals. A meta-analysis focussing on birds, the taxon best represented in our dataset, suggests that global warming has not systematically affected morphological traits, but has advanced phenological traits. We demonstrate that these advances are adaptive for some species, but imperfect as evidenced by the observed consistent selection for earlier timing. Application of a theoretical model indicates that the evolutionary load imposed by incomplete adaptive responses to ongoing climate change may already be threatening the persistence of species.