Most arctic birds have precocial chicks that are covered in downy feathers and can run, hide and feed themselves within a few hours of hatching. This is an important advantage, because most arctic birds nest on the ground, leaving their nest very vulnerable to predators. A group of helpless offspring huddled in a nest makes an easy meal for a fox, wolf or weasel. Precocial chicks, however, can run and disperse amongst the tundra vegetation, making them harder to spot and ensuring that a whole clutch will not be caught at once. These chicks often have a "freeze" response to danger, running to hide and then staying completely immobile, vanishing against the vegetation.