Summary.The behavioural interactions among breeding Western Sandpipers (Calidris mauri) are described, and interpreted in terms of their adaptive characteristics.Upon arrival in early spring, males disperse onto territories through mutual antagonism. Territorial advertisement and maintenance is accomplished through display flights low over the ground, which are accompanied by soft buzzy vocalisations. These characteristics of display are related to the fact that this species occurs in dense, localised breeding populations, where longdistance communication is unnecessary.Because densities are high and feeding often occurs in communal areas off territory, violations of territorial boundaries are frequent. The result is an extremely high intensity of interactions among individuals. Chasing and physical combat between males is frequent and generally un‐stereotyped. Birds nesting on territories away from the feeding sites often fly high above the other territories, and in doing so avoid being chased.Western Sandpipers form monogamous pair‐bonds which are maintained until the young are ready to fly. The initial association of the pair is facilitated by a strong tendency to return to the previous site and by the advertisement of the males in which a specific vocalisation is given. Once the female is present on a male's territory, she is courted persistently by him and gradually begins to participate in his scraping displays. Pair‐bonds are re‐enforced by the close association of the pair during the pre‐nesting period and through a simultaneous preening display.An hypothesis is developed that the participation of both sexes in incubation serves primarily as a means of pair‐bond maintenance, acting to keep both parents present until after the young hatch. At this time, their presence is needed to provide protection for the precocial young agains