The common house sparrow (Passer domesticus) hopping about your feet at a cafe or train station may provide clues into the effects of temperature on body size in a warming climate.

Nineteenth century anatomist Carl Bergmann found that in closely related species of warm-blooded animals, the cooler the climate, the larger the body size. Bergmann’s rule explains why koalas in southern Australia are larger and heavier than their lithe counterparts in Queensland. In 2016, the phenomenon was also observed in birds.

Size variation determined by differences in temperature has been studied since Bergmann’s time, and researchers have suggested that it is cold temperatures that select for larger body size. However, a new study on house sparrows published in the journal The Auk: Ornithological Advances suggests that the reverse may be the case – with hotter temperatures selecting for smaller body size instead.

Samuel Andrew and his team Macquarie University in Sydney, Australia, captured and measured 1901 introduced house sparrows from 30 locations across Australia and New Zealand. The birds were weighed, and lower leg length measured.

The team discovered that summer maximum temperatures were a better predictor of body size than the winter minimum. In the three locations with the hottest summer temperatures, the results were most stark, with the sparrows 6% lighter in mass, and tarsal length 2% shorter than in the three locations with the lowest summer temperatures.

The paper’s authors note the possibility that the sparrows in hotter locations may be using behavioural adaptation such as nest location to reduce the exposure of developing offspring to high temperatures.

“Our work on this common species helps us to understand the adaptive responses of birds to a changing climate and their constraints,” says Andrew.

Andrew’s recent study on zebra finches, published in the Journal of Evolutionary Biology, also showed experimentally that warmer temperatures in laboratory conditions significantly reduced body size.

“If variation in body size is linked directly or indirectly to adapting to different climates, then body size could be useful for monitoring the extent to which bird populations are capable of adapting rapidly to changing climates,” says Andrew.

Some researchers believe it’s not so easy to extend Bergmann’s rule to predict the effects of climate change through time. Researchers from the Museum of Natural History in France reviewed the primary literature across a range of taxa and concluded: “The mechanisms behind this rule are still debated, and more studies are needed before firm conclusions can be drawn about the underlying causes of these changes in body size in response to a warming climate.”