Either these are serial killers in the making (it typically starts with animals), in which case there is not much that can be done to really make them understand the impact of what they have done.  If however, they are just stupid boys, who did something stupid, then perhaps they can be made to do community service for a year working with endangered animals and the scientists working to save them.  Then perhaps they might have some concept of why so many people are so upset about this.  

So.. awful...They are lucky it was not crows... they have a long, long memory for those who have wronged them...