Anecdotal evidence suggests that increased numbers of waterfowl are remaining at northern latitudes throughout winter in the Mississippi Flyway. A calculated weather severity index (WSI), based on temperature and snow data, determined that local mallard abundance decreases when a WSI of 8 is reached. In mapping the WSI 8 line, mallard movement can be estimated. A fifty year trend analysis of the climatic factors driving duck migration for various locations within the Mississippi Flyway was used to determine whether climatic shifts have occurred, finding that although there are sinusoidal temperature trends throughout those years, the past decade has a longer and overall warmer trend. In examining the role of El Niño Southern Oscillation, it was found that in La Niña there is a more severe WSI, and El Niño correlates with a less severe WSI. A neutral Oceanic Niño Index caused a very high or very low WSI (was inconclusive).