Spatial trends in morphology do not predict species’ responses to climatic warming in Australian birds

Allen’s and Bergmann’s rules describe trends towards larger appendage size and smaller body size, respectively, in warmer environments in endothermic animals. Though conceived on a spatial scale, a key prediction from these ecogeographical rules is that they can be extrapolated to temporal trends. Accordingly, appendage size has been observed to increase, and body size decrease, in response to climatic warming across many taxa. However, these responses vary across species, and the cause of such variation is not yet clear. In this study, we use data from Australian birds collected over a 161-year period (1857–2018) to examine whether changes in morphology through time are predicted by whether the species follow Allen’s and Bergmann’s rule gradients across space. We collected body size (wing length and body mass) and bill surface area from 2014 museum skins from 17 species and modelled variation in these traits across space and time. Overall, we found decreases in wing length across the latitudinal range (from south to north), as predicted by Bergmann’s rule, but these were not matched by changes in wing length through time. Body mass did show Bergmann trends through both time and space (decreases in mass in warmer climates and over time). Bill surface area did not show any evidence of increase in size with proximity to the equator, but we did find evidence for increases in relative bill surface area through time, as suggested by Allen’s rule in response to the climatic warming. When comparing the relationship between spatial and temporal morphological trends for each species, we found no association between the size of their morphological gradients across latitude with those over time. Our findings suggest that thermoregulatory adaptation throughout spatial ranges do not directly predict temporal changes in morphology, nor do they explain the cross-species variation seen in temporal trends.