Birds on the eastside move up when things green up and insects start to show up, but always a certain few flocks that never really move. They're motivated by food, water and sex (sex for only a short period) Birds and animals don't move just for the heck of it, they move when they have to or food and water are in better supply. Snowline doesn't have a lot to do with it, some years when it's really cold with little snow the birds move down simply because the cold weather has killed the greenery and insects off up higher. Following or hunting the snow line only comes in to play when its coincidental with things greening up. Flocks break up when they separate to mate and bunch back up in the winter when feeding areas are limited.