(I) Data are presented on the kinds and amounts of nest-building responses that occurred in the different situations I observed in the laying and incubation periods. (2) With a few exceptions all situations in which an alteration in some condition produced change in the frequency of rising and settling also produced change in the same direction in the frequency of sideways-buildings and vice versa. On the other hand the numbers of collecting trips were not significantly altered by any of the changes which produced alterations in settling and sideways-building. (3) In most of the samples of watches settling and sideways-building were significantly positively correlated and performance of one response tended to be closely preceded or followed by performance of the other. There was virtually no correlation or temporal association between these two responses and collecting. (4) The majority of collecting trips were performed when partners were together on the territory. Situations which produced extreme increase in rising and settling produced a small number of collecting trips by gulls alone on the territory. Settling and sideways-building were by no means confined to situations when the mate was present although presence of the mate increased the quantity of these responses. (5) It is argued that the term 'displacement activity' is not helpful, and indeed misleads, when applied to the nest-building of Black-headed Gulls in the incubation period.