I don't know if you ever buy stuff on eBay but if you do then there are many of the sticky repellents like that TANGLEFOOT BIRD REPELLENT among others that if you would put some duct tape to the sill first the apply the gel on the back of that, then when the bird leaves just pull up the tape and there will be little to no mess to clean up.
Please don't use Tanglefoot -- it really can mess up the birds. Try the hawk silhouette or something shiny and reflective. I take old CD's, tie a string through the middle, and hang them where they catch sun and wind. The reflection and movement scares most birds away.