Birds build nests to keep eggs and chicks safe and warm, but they may also adjust nest insulation in very hot conditions to reduce the risk of overheating. Some mammals, such as rabbits and ground squirrels, construct burrow systems in which the temperature stays relatively constant, so they can avoid the more extreme temperatures that prevail above-ground. And termites – the master builders of the insect world – make mounds with their own solar-powered air-exchange system to control the temperature, humidity, and concentration of oxygen and carbon dioxide inside the colony’s nest.