Fig. 2: Morphology of northern populations relative to southern populations of 30 shorebird species.

Effect of the northern population is shown for relative bill length (A), wing length (B) and body mass (C); points above the dotted line indicate larger measurements in northern populations. Bill, wing and log-transformed mass measurements were scaled and centred, so effect sizes are comparable across species. Points represent the estimates for the effect of location derived from linear mixed models. Error bars show 95% confidence intervals. Effects are shown for species with different migration behaviours (migratory, nomadic/partially migratory, and resident) and foraging methods (probing and visual). Estimates controlling for sex differences in morphology are shown for sexually dimorphic or dichromatic species. Sample sizes are shown for birds caught at each location (southern observations | northern observations). Arrows indicate predictions based on the thermoregulation hypothesis (shown in Box 2). Average effects for migratory, nomadic and partly migratory, and resident species are: 0.45, 0.17 and 0.59 for bill length; −0.25, −0.38 and −0.90 for wing length; and −0.58, −0.36 and −1.22 for mass. See Supplementary Figs. S3–S5 for boxplots showing raw data.