Construction was first observed at 7:45 a. m., on June 12, 1939, when a female Yellow Warbler was seen carrying a tuft of plant-down into a small boxelder sapling. Upon examination, a mass of plant-down about one and one-half inches in diameter was found at a measured height of two feet three inches from the ground in the fork of the sapling. During an hour of observation the female continued to carry plant-down at intervals of about four minutes although once it did not bring any material for twenty minutes. At noon the plant-down mass had increased to about three inches in diameter and was more compactly pushed into the fork. By 6:45 p.m., there were many strands of plant fibers and grasses woven around and through the plant-down in such a way as to wrap and bind the plant-down around the small twigs of the fork. The nest was just assuming a cup-shaped structure. The female was now bringing large loads of a mixture of grasses and plant fibers and working at a rate of about one trip every four minutes. The first day's building was completed at 7:55 p.m. The nest was now partially surrounded with woven plant fibers and grasses with a slight formation of a rim. On the second day the work continued and the "rim consisted of plant fibers and grasses woven partly into the original down but mostly into the sides and around the top. At 6:45 p.m., the nest appeared completed with a well-formed cup, plant-fiber and grass rim, and a plant-down floor." The third day was partly rainy and little was accomplished but "by 8:00 on the fourth morning, the plant-down inside the nest was smoothed out and contained a few strands of fine grasses. * * * During all the observations on the building of this nest the male at no time was seen to bring any nest material. However, since there were many hours during the day when no observations were made, it is possible that he might have helped at some time. * * * At 6:30 a.m., the following day, one egg was found in the nest. * * * The dates of the beginning of construction and the dates the first eggs were laid were obtained for two other nests, and the time which elapsed in both cases was four days."