Biologists have known for a long time that animals living in colder climates tend to have larger bodies, supposedly as an adaptation to reduce heat loss. However, understanding how temperature affects animals has gained new importance thanks to climate change. A new study from The Auk: Ornithological Advances uses European House Sparrows, which have spread into a variety of climates in Australia and New Zealand since their introduction in the mid-19th century, to show that this trend in birds might actually be due to the effects of high temperatures during development—raising new alarms about how populations might be affected by global warming.