If i remember right one can say "Officer I was shooting at a Clay Pigeon and did not see the little model airplane".
There are 2 parts to the crime, Mens Rea and Actus Rea believe (Unless this is a strict liability crime which i dont believe it is in which case Mens Rea is not important)
Actus Rea - The criminal act - Shooting at the bird -- Guilty here
Mens Rea - The Mental State - Might get them on something else but they cant get intent if they are having a shooting competition and their bird gets shot down.
Also, flying at a low altitude can be considered a violation of ones curtlidge, if the normal citizen can not reasonably get there. The normal citizen can not fly that low possibly..
If there is an actual Lawyer Here and not a CJ student you are welcome to correct me. I just see this as a pretty good lawsuit. SHARS liable in my opinion.