You haven't had a bird s**t on your car, until one of the local Ostriches in my area have done the deal.
14/4/11 f**king Birds by Harpham y0, on Flickr
That was just on half the bonnet.
clay bar helps, scratch x, tar remover
If it has etched the paint, a machine polish is the only way to really get rid of it - you can try hand polish etc but it wont have the same effect.
I hate bird crap
My old clio got coated in it just after I had washed it (sorry for the big pic):
Keep some of wet wipes or those Autoglym wipes in your car. Having seen the damage it does on my sister in laws black golf it may sound ocd but get it off as soon as you can in 1st place.
The only way to get rid of the etching completely will be to cut it back. Something like auto glym polish or scratch-x may be enough otherwise a DA or rotary may be required with some cutting compound. Making sure you have a good coat of wax on will always help.