Ice, Snow, and Pesty Critters
This drawing is an excellent illustration of ice damming and the amount of damage that can be incurred in your home. When a large amount of snow accumulates on the roof of a building the snow will act as an insulator to the outside cold and heat loss from within the building. A lack of enough insulation and poor attic ventilation will warm the roof and thawing will result. When this melted snow runs down the roof and reaches the roof overhang it begins to freeze because there is a lack of heat and freezing begins.
To keep the damming process from forming roof heat tape can be installed in the spring.
If they're already established just leave them alone and let Mother Nature run its course. The world will be a better place to live.
They do not like anything hanging or swinging over their heads and will find a better place to nest.