SummaryBreeding biology of Shelduck was studied in England on the Isle of Sheppey, Kent. Virtually complete separation of breeding and non‐breeding sections of the population occurred and non‐breeders gathered at two feeding areas which were also the foci of the winter population.Pairing of first spring females usually occurs in the non‐breeding flocks and bonds are then of long duration.There is an excess of males throughout the year and during the winter, particularly in February, these attempt to pair up by breaking into established pair bonds. Some probably succeed.Flocks of adults in early April were demonstrated to be potential breeders: such flocks appear to provide mutual stimulation. Smaller, socially discrete groups of breeding adults were discovered and named “communes”. These nested together, held adjacent territories and remained in contact throughout the breeding season, even when most pairs had lost nests. They are considered to be the basic social unit of breeding populations, perhaps of all populations.Breeding birds were unevenly distributed such that the density of all nests was one per 230 acres, but the maximum was 6 per acre.Average clutch size of 10·1 and mode of 8 were found from nest records, compared with 8·8 and 9 respectively deduced from broods of ducklings less than three days old. Egg laying began at the end of April and lasted about 81/2 weeks.Incubation spanned some 10 weeks from early May; individual periods averaged 30 days. An automatic recorder showed that one female spent 87% of the available time incubating.Shelduck were found to be single brooded and no evidence of re‐nesting occurred.The brood period started in early June and lasted about 9 weeks. Crêching was considered to result mainly from desertion by parents, but over‐crowding was also a factor. Fledging takes approximately 61/2 weeks.In 1962 and 1963 approximately 54% and 59% respectively of the summer population attempted breeding. Success in getting broods to water (called duckling success) varied from 23·9% to 73·8% in the period 1960 to 1963. This represented a breeding success of 16% to 49·2%. Duckling losses appeared to be heavy.Shelduck generally breed first in their second spring, but some males may not breed until they are almost five years old.