Australian birds track climate warming over decades via shifts in bill morphology

Abstract

Global warming is widely predicted to drive adaptive responses in species' morphology, yet such responses have rarely been demonstrated to track changing climate through time. We investigated associations between avian bill size and changes in seasonal climate extremes using museum specimens of 57 Australian passerine species (Meliphagides). We employed location-specific climate data from the 4119 sites where specimens were collected to test whether change in the frequencies of summer (≥35 °C) and winter (<5°C) extremes over the preceding 20 years of each specimen's collection date was associated with bill size variation as predicted by thermoregulatory hypotheses. Rapid warming in summer and winter was associated with bill size variation in accordance with the bill's role in heat exchange. Our study provides critical evidence that bill size is tracking climate change through time in response to changing thermoregulatory demands. A critical future question is whether such changes facilitate species persistence.

Fig. 1. Collection locations of museum specimens used in the study.

Included are 4119 individuals from 57 species of Meliphagides sampled across the temperate zone (shown in yellow) as defined by the Köppen-Geiger climate classification. The dots are coloured according to the collection year, and dot size represents the bill size of individuals relative to their body size for visual comparison. Boundaries of IBRA regions are shown in grey.

Fig. 2. Climatic conditions experienced by individuals included in the study.

Histograms show the number of specimens from locations that experienced an increase in a days ≥35 °C, and a decrease in b days 5 °C, and a decrease in c primary cover (km²) per year. The number of individuals that experienced a change in the opposite direction or no change are shown in grey.

Fig. 3. Association between relative bill size of Meliphagides and rate of change in climate extremes and primary cover over 20 years.

a, b illustrating model predictions for relationship between bill size and Δdays ≥35 °C in interaction with summer rainfall; c, d bill size and Δdays ≥35 °C in interaction with mean maximum summer temperature; e, f bill size and Δdays < 5 °C in interaction with rate of change in primary cover (km²) across 57 species. Regression lines show predicted bill size response to the range of values between the 2nd and 98th percentiles of observed climate values of interacting variables. Black dashed line in (b) shows amount of rainfall below which the birds responded to summer warming by increasing bill size; d mean maximum summer temperature above which bill size starts to decline with increase in rate of summer warming. The y-axis has been truncated to enable visualisation of model predictions. (Supplementary Material Fig. S2 shows responses to the full range of values of variables along the x-axis and includes all data points).