Depends upon the severity of the marking, but using an automotive claybar will bring out most bird markings. If that fails, you'll probably need a fairly aggressive scratch removal polish to cut down into the clearcoat a bit.
If you've never used a claybar, most kits will come with their own clay lubricant. If it doesn't or if you are able to find just the claybar itself, you can use any "quick detailer" spray or even water with a very small amount of soap as the lubricant. Just keep the surface you're working on wet and never never never drop the claybar, if you do throw it away.