Using different body size measures can lead to different conclusions about the effects of climate change

Aim Declining animal body size has been proposed as a general response to increasing global temperatures that should be observed across a broad biogeographical scale. However, published studies have shown large variation in both the magnitude and direction of body size trends. We aim to investigate how the way body size is measured (body mass, structural size, body condition) may contribute to differences in body size trends between studies. Location Semi-arid Australia. Taxon White-plumed honeyeater (Ptilotula penicillatus). Methods We studied two separate populations of P. penicillatus over 30 years to investigate the associations between weather and body size. We first investigated how body mass has been affected by weather conditions at each site and then determined how these same local weather conditions affected the constituent components of body mass (structural size and body condition). Results The magnitude and direction of weather effects differed with the measure of body size used. Average structural size (wing length) increased with increasing temperatures while average body condition decreased. As body mass is a composite of structural size and body condition, the magnitude and direction of body mass trends was affected by trends in these two other traits. For example, differences in temperature effects on structural size between our two sites led to clear differences in body mass trends. Main Conclusions Trends in body size will be strongly affected by the choice of body size measure used in analyses. Change in body mass can be particularly difficult to interpret as it will be a composite of changes in both structural size and body condition. Our results indicate that it is difficult to compare studies using different measures of body size. Using consistent measures of body size will be important to better understand the general effects of climate change on body size.