This nesting study of the Red-eyed Vireo was made in the Douglas Lake region of northern Lower Michigan. The Red-eyed Vireo was an abundant species in the beech-maple-pine and aspen-red maple communities, and from the tree tops to the lower fringe of the upper canopy. The Brown-headed Cowbird was a common species in the communities and a frequent social parasite on the Red-eyed Vireo. The niche requirements of the vireos included (1) an open, deciduous woods with several tall trees, (2) scattered clusters of saplings, (3) a small clearing or opening within the woods, (4) loose, fibrous bark for nests, (5) supply of larvae for food. Most of the viroe population probably arrived in the Douglas Lake region between May 15 and May 25. Nest-building probably commenced the first week in June and egg-laying started the second week. The size of the vireo territories averaged 1.2 acres. Males ranged at will throughout their territories. Territorial boundaries disintegrate by the middle of July when nesting probably comes to an end. Nests, as a rule, were somewhat centrally located within the territories. Several territorial disputes among vireos were witnessed. Individuality played a role in the aggressiveness of the vireos toward other bird species. Defense activity increased with the different stages of nesting. During nest-building, defense was slight. Just after fledging of the young defense reached its peak of intensity. The female was more aggressive than the male. Singing periods of the male were determined. The Red-eyed Vireo was apparently single-brooded. Courtship was observed during nest-building. Most of the nests (97.12 per cent) were in deciduous trees, preferably red maple (71.43 per cent). The heights of nests ranged from 1.33 feet to 26 feet (average 7.64 feet). Nests were suspended from the lower branches of saplings at varying distances from the frunk (0.0 to 9 feet). There was a correlation between size of tree and the distance from trunk to nest. ....