Information on waterfowl survival during the overwintering season (i.e. autumn and winter), when hunting seasons occur, is important for making harvest management decisions. However, the relationship of overwintering survival to hunting season structure, weather and body condition are not well understood. We measured survival of 235 radio-marked adult female and male mallards Anas platyrhynchos along the South Platte River corridor in northeastern Colorado, USA, during the overwintering seasons of 2005/06 (pilot year), 2006/07 and 2007/08, and we determined the primary factors affecting survival. Hunting was the most important factor affecting survival. Of mortality, 67% were direct results of hunting, and survival was lower during hunting periods compared to non-hunting periods. Within hunting periods, survival was lowest during the first 2-3 weekends of the hunting periods. During the seasons 2006/07 and 2007/08, survival of radio-marked mallards was monitored during September-February. The estimated survival was 0.65 (95% CI = 0.50-0.78) for females and 0.54 (95% CI = 0.39-0.68) for males during 2006/07, and 0.55 (95% CI = 0.40-0.69) for females and 0.42 (95% CI = 0.28-0.58) for males during 2007/08.We did not observe a strong correlation between body condition index and survival (¯β = 0.36, SE = 0.43). Accumulated snowfall and daily minimum temperature were unimportant variables for predicting survival. Of hunting recoveries, 89% occurred in our study area, and 15% and 18% of radio-marked mallards went missing during 2006/07 and 2007/08, respectively. Our results suggest that split hunting seasons are an effective management tool to increase hunter harvest and affect overwintering survival. Given a set bag limit and season length, managers may be able to increase hunter harvest by: 1) having hunting periods of at least three weeks in length, 2) including as many weekend days (i.e. Saturdays and Sundays) within hunting periods as possible and 3) interspersing hunting periods with non-hunting periods of at least 2-3 weeks.