Non-consensual body size response to climate and primary production in common songbirds: habitat and thermal preference matter

Nicolas Dubos (1,2), Isabelle Le Viol (1), Alexandre Robert (1), Céline Teplitsky (1), Olivier Dehorter (1), Manon Ghislain (1,2), Romain Julliard (1) & Pierre Yves Henry (1,2)

1 Centre d’Ecologie et des Sciences de la Conservation (CESCO UMR 7204) Centre de Recherches sur la Biologie des Populations d’Oiseaux, Sorbonne Universités, MNHN, CNRS, UPMC, CP51, 55 rue Buffon, 75005 Paris, France. Email : nicolas.dubos@mnhn.fr

2 Mécanismes adaptatifs et évolution (MECADEV UMR 7179), Sorbonne Universités, CNRS, MNHN, 1, avenue du Petit Château, 91800 Brunoy, France.

Is body size decline a universal response to climate change? While there is some evidence for size decline in endotherm species over the last decades, the universality of this pattern was questioned, as species are exhibiting either size decline, increase or no change. The underlying mechanisms of body size change remain poorly understood, and it is not clear whether body size change is directly related to changes in ambient temperature (thermal constraint) or mediated by food availability (resource constraint). Based on field  biometric  data  (n  >  37000),  we  tested  the  response  of  juvenile  wing length  and  body  mass  to temperature, rainfall and remotely sensed primary production (NDVI) for 41 common songbird species across France between 1989 and 2013 (248 sites, 25 years). Our results indicated that (1) both climatic and primary production fluctuations were significant drivers of juvenile body size in some species and (2) there was  no consensual response between species to  the tested variables. We tested which species traits explained this differential sensitivity. Temperature affected wing length positively in thermal specialists and habitat specialists, while thermal and habitat generalist species were the least affected by temperature change. Besides, body mass was positively related to NDVI for thermophile and large species. We discuss the complexity of the mechanisms underlying body size change and the importance of accounting for between-species variability in understanding species response to climate change.

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