SUMMARYLANGHAM, N.P.&K. HULSMAN. 1986. The breeding biology of the Crested TernSterna bergii.Emu 86: 23–32. The breeding biology of the Crested TernSterna bergiiwas studied on One Tree Island, Capricornia section of the Great Barrier Reef Marine Park, Australia. The dense nesting habit with its associated adaptations resemble those found in related species of terns. Clutch size was 1.01 with a mean incubation period of 28 days. Laying was spread over 53 days, but individual subcolonies were more synchronised (18±11.5 days). Hatching success in 1979/80 was 69%, most failures being attributed to predation by the Silver GullLarus novaehollandiae.Smaller subcolonies with proportionally more nests on the perimeter were subjected to greater predation of eggs than were large subcolonies. Overall, there was a decline in hatching success with season. Fledging success was high (85%) with most mortality in 1979/80 being the result of a tropical cyclone. Gulls stole from a third to half of the fish brought to the colony in January 1982. Fledging period varied between 35–43 days. The growth constant (K) was about 0.120, compared with 0.165 for the north temperate Sandwich TernSterna sandvicensis.These results are discussed with particular reference to Silver Gull predation.