An intensive study of 4,085 nests of the catbird (Dumetellacarolinensis) was made in Michigan, Ohio, Kentucky, and Ontario during the 27-year period 1934-1960. Emphasis was on habitats, nest sites, nest materials, clutch size, incubation and behavior. Most of the study was conducted in Oakland County, Michigan, where 87.5% of the nests studied were located (45 separate locations). Another 5.8% were found in 15 other Michigan counties, and the remaining 6.7% at six locations in Ohio, Kentucky, and Ontario. The catbird is abundant in southern Michigan during the nesting season, and it occupies some habitats which were not available to it before settlement by man. The increase of suitable habitats since colonization by early settlers has been due almost entirely to lumbering and agricultural activities, and more recently to urban expansion. The catbird nests in a great variety of habitats, largely irrespective of the tree and shrub associations involved, provided "edge" is present and the density is suitable. The catbird has invaded man''s dooryards with great success and occurs there in greater concentrations than in many wilder situations. Several spp. of shrubs introduced for landscaping purposes have been of considerable importance in attracting the catbird to the vicinity of buildings. Undoubtedly, the catbird has been responsible for the spread of some fruited spp. to many wild areas far from the original plantings. The catbird is strongly territorial in its nesting requirements but apparently shows greatest pugnacity in the immediate vicinity of the nest site. Song plays a dominant role in defense of territory and usually there is little physical combat involved. It appears to be a solitary nesting bird. Because of the birds generalized habitats, its nests are, in this study, found in a great variety of vegetation; the nest is at an average height of 5.2 feet. The sources of nesting materials are usually near the nest site. The catbird is usually double-brooded, nesting over a period of 2 months (May 5 to July 5). The time required for the construction of 19 nests observed from beginning to completion averaged 5.1 days, with extremes of 3 and 7 days. Periods between completion of the same 19 nests and the laying of the 1st eggs averaged 2.1 days, with extremes of 1 and 3 days. All except one of 10 eggs laid in 6 observed nests were deposited before 900 a.m. sun time. Data on 616 clutches for 14 nesting seasons are included in this study. Incubation period as determined from 30 nests, beginning at the time the last egg was laid, averaged 13.4 days, with extremes of 12 and 15 days. The hatching time of 42 young in 23 nests was recorded. One young hatched before 12:00 noon, all others between 12:00 noon and 8:00 p.m. sun time. The average nestling period for catbird young in 17 nests was 11.3 days with a range of 9 to 15 days. Of 34 young in 11 nests, 8 young left 2 nests about 12:00 noon; all others flew between 5:00 and 8:00 p.m. sun time. Catbird young do not remain long in the vicinity of the nest once they are fledged. Only one young of more than 1000 nestlings banded has been recaptured later in the nesting area. Within the region of this study the catbird is subject to several predators. The red squirrel, chipmunk and raccoon appear to be most destructive. Cowbirds lay eggs in catbird nests, but most eggs are removed by the hosts. I have found only one record of catbirds raising a cowbird young.