Most fist wave migrations happen due to a ducks "internal clock" similar to that of a Salmonid species returning to the river from the blue waters of the ocean. After that, most duck migrate due to food and feeding needs.
Hens and a majority of younger birds will start to head south due to the freeze, this is because they are more "open water" type feeders, as they cant digest grains (rice, barley, millet, and corn) as well as older birds.
Next you will see a big push, the high point of the migration, kick in as the heavy snows start to cover up these grain fields and feeding holes that the older birds are using. When the snow starts to cover the barley and millet, oats and corn up north, the birds start to move to find food.
A ducks life is spent focused on three things. Eat, Survive, and #*%@
If needs two of these three things to be a happy camper...
Give them food and "friends" and they will let their guards down.
But that's just my two sense on the subject anyhow.
