Risk factors for predation attempts by Peregrine Falcons (Falco peregrinus) were examined in Semipalmated Sandpipers (Calidris pusilla) during fall staging in the Bay of Fundy, Canada. Odds of attacks may be expected to vary with a host of factors such as time of day, time of year, spacing within groups and weather conditions. During the ebbing tide, 65 surprise attacks by falcons were recorded over 596 observation periods. Two factors emerged as risk factors for predation attempts by Peregrine Falcons on Semipalmated Sandpiper flocks during four consecutive fall staging periods. Falcons initiated attacks more often on denser flocks and later during the migratory stopover. Year, time of day and weather factors did not influence attack rate. These findings may reflect lower anti-predator vigilance in denser groups and the greater numbers of sandpipers available in the later stages of stopover. Fitness consequences of these attacks remain to be established.