During territorial behaviour and pair formation willow ptarmigan cocks and hens use several different calls. Cocks use mainly a “flight call” a “ground call” and three different “threat calls”. Hens give similar calls to cocks. It is suggested that the structure of the calls is well adapted to (1) transmit their possible infomation content over long distances, and (2) make localization easier for birds during the territorial display periods, which take place mainly in poor light at dusk and dawn. We also suggest that the cock and hen calls express different degrees of aggressiveness, and that hen calls, in addition to attracting cocks, function as territorial “keep out” signals to other hens.