Males of many polygynous species compete for access to fertile females without providing them with resources other than sperm and without investing in care for the offspring (male dominance polygyny)(1-4). In such systems, local competition for access to females is intense and typically only a few males obtain matings, leading to strong sexual selection(2,4,5). Sampling multiple breeding areas could then provide a mechanism for males to increase their chances to reproduce. However, little is known about such sampling behaviour and about the spatial scale at which males compete(6-8). Here we show that most males of a migratory, polygynous shorebird, the pectoral sandpiper (Calidris melanotos)(9,10), that arrived at a known breeding location in northern Alaska(9) subsequently moved through a considerable part of the entire species' breeding range (up to 13,045 km in a four-week period), sampling as many as 23 additional potential breeding sites. Our data suggest that males do not have a final breeding destination after migration from their wintering quarters, but make nomadic movements that are probably not a consequence of breeding failure. Tenure, the duration of stay at a site, correlated strongly with the number of breeding females at the site, suggesting that decisions to leave are dependent on local mating opportunities. Nomadic movements may allow males to display and sire offspring at multiple sites within a single breeding season. Sexual selection may then favour high-performance males that are able to reduce sleep to compete locally(9) and to fly long distances between breeding sites, leading to a population with unrestricted interbreeding and without local adaptation and speciation.