About one-seventh of the bird species in the Arctic have young that are naked and helpless when they hatch. Although this is a disadvantage in the cold Arctic climate, these birds – such as snow buntings and sparrows- protect their young from the cold by brooding them constantly until their feathers grow – one of the parents sits on the nest, sheltering and warming the chicks. Unlike many tundra nesters, these birds weave nests from grass and moss and small twigs, and then line them with warm insulating materials such as muskox wool, mammal hairs or feathers. This combination of good nests and constant parental care allows vulnerable chicks to survive even the chilliest conditions.