Moller A.P. (1981) Breeding cycle of the Gull-billed Tern Gelochelidon nilotica Gmel., especially in relation to colony size. ARDEA 69 (2): 193-198

The breeding biology of the Gull-billed Tern Gelochelidon nilotica was studied in different Danish breeding colonies. Clutch size proved to be larger in larger colonies possibly due to social stimulation, and perhaps to the better quality of the birds in medium-sized and large colonies. Synchronization of laying occurred in larger colonies, which shortened the length of the laying period, and with it the length of the birds' stay in the colony area. In Denmark, the Gull-billed Tern is found at the northern border of its range. It is suggested that the timing of the arrival on the breeding grounds is critical to the terns, and especially to the females which have to produce eggs. Courtship feeding may be of great importance, especially if food is scarce in spring. Temperature may influence the amount of food available and, with it, the length of the pre-laying period, the period being longer with an early arrival. The average clutch-size is largest in the middle of the breeding season, when most birds start laying, with the result that most young are produced when food is plentiful. The presence of Black-headed Gulls Larus ridibundus near breeding Gull-billed Terns may force the terns to breed early so that the gulls are still present on the breeding grounds. The recorded breeding success of 69.1% was higher than that in other tern species.