If its a good area for them I wouldn't worry to much, they should be back. Fall and winter if the weathers not to bad the large flocks can move around quite a bit. Once spring breakup occurs they will spread out more to cover more ground. I think the biggest thing that would affect population is the cold wet springs. Hens can get the 'wet dog' syndrome (smell) and attract predators when nesting, and very young ones can die from getting cold (and wet).