Birds do not work harder to breathe.

An efficient one-way airflow system is essential for bird flight. Extracting more oxygen per breath than mammals, birds can meet the costly fuel demands of flight. However, despite the gains achieved by maximising the amount of oxygen passing through their lungs, the metabolic cost of breathing for birds could be high. Birds were thought to have to work against the weight of their flight muscles loading down the sternum as they breathe, increasing the exertion of breathing even during rest. Yet, no one had ever successfully measured the direct cost of breathing for resting birds. Intrigued by the possibility that highaltitude species may also have evolved special adaptations to minimise the cost of breathing in thin air, Julia York, from the University of British Columbia, Canada, and colleagues from Canada, USA, Peru and Australia, collected 11 species of duck (including teal and pintails) from high-altitude locations (3812 m at Lake Titicaca in Peru) and nearer sea level (∼1260 m in Oregon) to measure how much effort it takes birds to breathe.