Ditto's
Ultra violet light will fade and deterorate the pigment and fiber of the mount. The additional heating and cooling of those areas expand and contract the skin surface and can cause cracking in mounts that are not professionally tanned and built on very stable platforms.
There are UV protectants that can be applied to the hair of the mount. and there are UV protective films you can put on the windows but these help only slightly with the heating and cooling issues. (Fully reflective films can be used that will negate both problems but darken the windows and keep the neighbors from enjoying your mounts!
Mike
www.mikescustomtaxidermy.com