The incubation period seems to run from 21 to 23 days. A nest found May 31 with the complete set of four eggs was hatched on the morning of June 21. Another nest containing four eggs, from which the old bird was flushed, was found on June 2 and hatched on June 25. The first young were found on June 21. Contrary to their habits when there were only eggs in the nest, the mothers now showed great concern for their young. At one time Murie caught some newly hatched young, and holding his hand containing them extended on the ground, induced the old bird to come up and brood the chicks. She was so tame that he caught and banded her without difficulty. The male seems to take no part on the incubation or care of the young. He was often seen to join a hen driven from the nest, but only for purposes of courtship, as he would start booming immediately and chase her about. Before the eggs began to hatch, male birds seemed to disappear from the tundra. There was never more than one bird seen with the young. Thirty days seemed to be about the time necessary for the chicks to mature, as by July 20 fully fledged young were seen commonly about the tundra. Mr. Buturlin (1907) says: When I approached the breeding ground the old birds flew to meet me, one after another, and wheeled around uttering low tremulous notes of various kinds. These calls were evidently meant for the young and had different meanings. When the female is with them (and you must sit watching for an hour or more to observe this), the little ones are somewhat shy and take refuge under her. If you make the slightest movement she flies up, uttering the usual _kirip_, and kicks the young forwards, never backwards, until they tumble head over heels 5 or 6 inches away. There they lie as if dead, but with open eyes, and the mother flies around uttering a low tremulous _kirip, kirip, trip, trrrrrr_, evidently meaning "lie quite still." Then she alights near the young and runs about feigning lameness, while trying in every way to make you attempt to capture her. If, however, you keep quite quiet she becomes reassured, approaches near to where her young are, and utters with tender modulations, _day-day-day, day-day-day_, which means evidently "all right, come here." Then the chicks commence to chirp _peep, peep, peeyp_, and run to their mother. On one occasion I observed all this at a distance of about 10 paces, and once I was only about 3 paces from them. The downy young know their mother's call _day-day-day_ so well that on one occasion a young bird, which I was taking home in my butterfly net, when it heard a female call quite close to me, climbed out of the net to rejoin her. Mr. Brandt in his manuscript notes writes: