heat light in the winter?

Usually unless you have a lot of snow and/or rain Muscovy will do well with a shelter to get out of the elements & a layer of bedding to keep their feet warm.  Heat lamps aren't usually needed.  If you have a lot of snow which prevents easy feeding/watering and the like a way to pen them indoors is probably a good idea.  I still wouldn't use a heat lamp but would make sure that the building was as draft-free as possible.  Ducks usually stay warm (and bouyant!) by trapping air against their body underneath the feathers.  If their feathers are in good condition they should be able to handle *most* weather fairly well.  Very cold temps might cause some frostbite to caruncling so if I had drakes with a lot of caruncling and a long winter of very cold temps I might then consider something to help prevent frostbite.