Well, they enjoy using them to line their nests. According to this - birds have figured out that the end of a smoked cigarette can be used to repel parasitic mites. It seems the nicotine in discarded butts is a natural bug repellent and nests that contained used smokes had the fewest mites.
There are also a host of birds that eat discarded cigarette butts and die.
So, no excuse to not pick up your butts!
Said the ex-smoker.
Thanks to Paul Baicich for passing this along to us...