Climatic data
To characterize summer conditions at sites of origin, we selected the following five bioclimatic variables from the WorldClim database (Fick and Hijmans 2017): annual mean temperature, maximum temperature of warmest month, mean temperature of warmest quarter, annual precipitation, precipitation of warmest quarter (see Fig. 2Aand B , and Table S2 for regional ranges). For all sites, the warmest quarter of the year occurs between June and September (Fig. 2A and B ).