Reduced avian body condition due to global warming has little reproductive or population consequences

Body mass, body size and reproduction data were collected as part of the Dutch Constant Effort Site (CES) program, which covers 80 sites across the Netherlands, spanning 21 years (1994-2014; see Appendix 1 Fig S1 for map). The CES-project follows a standardised protocol where birds are captured using mist nets from the 13th April until the 13th August every year (Robinson et al. 2009). Most sites were sampled 12 times per year (range 9-12). Captured birds are ringed and morphometric measurements taken, including body mass (grams) and wing length (maximum chord measurement; Svensson 1992). The sex and age-class (juvenile or adult) are classified based on plumage (wherever possible). For all species we consider juveniles to be less than one year old and born in the year of capture. Independent annual population growth rates were calculated using abundance count data from the Dutch Breeding Bird Monitoring Program (BMP) which has been running since 1984. This bird counting methodology uses fixed study plot territory mapping, a bird counting methodology that produces reliable estimates of total bird abundance compared to point/transect counts, and makes year-to-year comparisons possible at the site level (Van Turnhout et al. 2010). Study plots are visited 5-10 times per year, with every visit covering the whole plot. We calculated the weighted mean abundance for each of the CES sites by averaging the 5 closest BMP sites that had the same habitat type (typically within a 15km radius of the CES sites). Estimates in this data set are calculated per the methodology in the article. Please see the metadata tab for specific information.