Australian songbird body size tracks climate variation: 82 species over 50 years

Abstract

The observed variation in the body size responses of endotherms to climate change may be explained by two hypotheses: the size increases with climate variability (the starvation resistance hypothesis) and the size shrinks as mean temperatures rise (the heat exchange hypothesis). Across 82 Australian passerine species over 50 years, shrinking was associated with annual mean temperature rise exceeding 0.012°C driven by rising winter temperatures for arid and temperate zone species. We propose the warming winters hypothesis to explain this response. However, where average summer temperatures exceeded 34°C, species experiencing annual rise over 0.0116°C tended towards increasing size. Results suggest a broad-scale physiological response to changing climate, with size trends probably reflecting the relative strength of selection pressures across a climatic regime. Critically, a given amount of temperature change will have varying effects on phenotype depending on the season in which it occurs, masking the generality of size patterns associated with temperature change. Rather than phenotypic plasticity, and assuming body size is heritable, results suggest selective loss or gain of particular phenotypes could generate evolutionary change but may be difficult to detect with current warming rates.

Keywords: Bergmann's Rule; body size; climate change; heat exchange; metabolism; starvation risk.

Figure 1.

Temporal trends in (a) body size (wing length) and (b) mean annual temperature across 50 years; (c) mean summer maximum temperature and (d) mean monthly rainfall (1970–2010) for 82 species of passerine bird in the Meliphagides. Each line represents a single species, and the order of species is the same in each panel. We calculated mean temperature, mean summer maximum temperature and mean monthly rainfall for each year for each species, based on values for each month, extracted for all cells in the distribution of each species. Solid green dots represent significant change in body size; green circles indicate trends. (Online version in colour.)

Figure 2.

Association between the change in structural body size (wing length) and change in mean annual temperature, given mean maximum summer temperature for 82 species of passerine bird from Meliphagides (sensu [24]), formerly superfamily Meliphagoidea. (a) Three-dimensional representation of the phylogenetic generalized least-squares model predicted relationship; (b) two-dimensional representation showing species data points, PGLS regression lines for the relationship between change in body size and mean summer maximum temperature showing species that experience different degrees of temperature change with PGLS model regression lines for 0, 0.004, 0.008, 0.012, 0.013, 0.014. 0.015 and 0.016°C increase in mean temperature; (c) similar representation of relationship between change in body size and change in mean annual temperature showing species that experience different mean summer temperatures: model lines represent relationship at 18, 23, 28, 33 and 38°C (colours as represented in legend). (Online version in colour.)