Viktoriia Radchuk, Thomas Reed, Celine Teplitsky, Martijn van de Pol, Anne Charmantier, Christopher Hassall, Peter Adamik, Frank Adriaensen, Markus P. Ahola, Peter Arcese, Jesus Miguel Aviles, Javier Balbontin, Karl S. Berg, Antoni Borras, Sarah Burthe, Jean Clobert, Nina Dehnhard, Florentino de Lope, Andre A. Dhondt, Niels J. Dingemanse, Hideyuki Doi, Tapio Eeva, Jörns Fickel, Iolanda Filella, Frode Fossoy, Anne E. Goodenough, Stephen J. G. Hall, Bengt Hansson, Michael Harris, Dennis Hasselquist, Thomas Hickler, Jasmin Jasmin Radha, Heather Kharouba, Juan Gabriel Martinez, Jean-Baptiste Mihoub, James A. Mills, Mercedes Molina-Morales, Arne Moksnes, Arpat Ozgul, Deseada Parejo, Philippe Pilard, Maud Poisbleau, Francois Rousset, Mark-Oliver Rödel, David Scott, Juan Carlos Senar, Constanti Stefanescu, Bard G. Stokke, Tamotsu Kusano, Maja Tarka, Corey E. Tarwater, Kirsten Thonicke, Jack Thorley, Andreas Wilting, Piotr Tryjanowski, Juha Merila, Ben C. Sheldon, Anders Pape Moller, Erik Matthysen, Fredric Janzen, F. Stephen Dobson, Marcel E. Visser, Steven R. Beissinger, Alexandre Courtiol, Stephanie Kramer-Schadt

• 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.