From April to September, 1945, and from April to June, 1946, a field study was undertaken on the natural history of the common black-headed grosbeak (Pheucticus melanocephalus) in the breeding season (Weston, 1947). The following discussion, related to the development of immature birds, covers but one phase of this study. Observations were in Strawberry Canyon, on the campus of the University of California, Alameda County, California. Eight nests were studied in detail, six in 1945 and two in 1946. The usual set of eggs in this species consists of three to four eggs. Occasionally sets of two and five eggs are recorded. A study of 192 nests from the literature and other sources disclosed a mean clutch size of 3.31 eggs. Incubation lasts twelve days as does the following nestling period. Data were gathered on the development of eleven nestlings
