Our bodies generate heat energy by digesting the food we’ve eaten during the day. The best way to prevent the loss of this warmth is to keep it from leaving our bodies too soon. That is why we wear varying layers of clothing, depending on the weather. Warm air, trapped between the layers, is unable to reach outside. Preventing energy loss in this way is known as insulation.
The eider duck employs the exact same method. Like many birds, its feathers enable it to fly and also keep it warm. It uses its soft and fluffy chest feathers in building its nest. This down protects the eggs and the emerging featherless chicks from the cold air. Since the eider’s feathers retain warm air, they exemplify the very best form of natural insulation. (Phil Gates, Wild Technology, p. 64)
Modern mountain climbers keep their bodies warm by wearing special costumes filled with feathers with high heat-retaining properties, similar to those of eider feathers.