The size of European House Sparrows in Australia and New Zealand may be influenced by high temperatures during development. Source: P. DevicheIt has long been understood by biologists that animals residing in colder climates are typically larger in size than animals that do not reside in colder climates, evidently as an adaptation to reduce heat loss. Now, a new study of European House Sparrows residing in a variety of climates in Australia and New Zealand indicates that this may actually be thanks to the impact of high temperatures during development, which raises new concerns about how these populations are impacted by global warming.

Capturing and measuring roughly 40 adult House Sparrows from 30 locations across Australia and New Zealand, researchers from the Macquarie University found that maximum temperatures during the summer (when the birds breed) better forecasted adult body size at the different locations than the winter minimum temperatures. This suggests, according to researchers, that excessive heat during development has a greater impact on a bird’s lifetime growth than previously suspected — raising alarms that the climbing summer temperatures associated with climate change could “drive down” the average adult bird body size, thereby potentially affecting the fitness of that population.

"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," said Macquarie University's Samuel Andrew. "Our work on this common species helps us to understand the adaptive responses of birds to a changing climate and their constraints, and this fundamental knowledge will help future workers and managers focus their work on other species and potentially identify those species most at risk from climate change."

"This paper is an important addition to a growing body of work that is changing our understanding of the relationships between climate and body size. The big question generated by these results is the extent to which the observed relationship is the outcome of adaptive evolutionary differences among sites as opposed to direct developmental responses to different temperatures. Interestingly, some of these same authors just published experimental evidence for a direct effect of temperature on growth in another bird species," added Whitman College's Tim Parker, an expert who was not involved with the research. "This is not a new idea, but it has been largely ignored by those who have assumed that most morphological variation in birds is due to evolved adaptive variation. We need more work on the direct effects of temperature variation on development in endotherms."

For more on the study, click here.