I've seen windows made from a variety of woods such as spruce, lodgepole pine, Douglas fir, and red cedar. I believe in older times they were made from white oak and I think I've seen some made from western hemlock too. I had a local friend make some for me out of red cedar that I supplied him and they turned out well and have held up well although cedar is the lightest and least strong of any wood mentioned. I bought some made from pine and I need to rebuild them.
If you find a good source for window weathering stripping post it as I will have to replace the ones that are on mine and I'm looking for a good source.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.