I agree with everyone else, maybe because I am a parent as many other guys here.
My mom used to be the same way. She bought my first airgun (Crosman pistol) when I was 13, but she didn't think I was going to be shooting cirtters. I respected her wishes but eventually a year later I was having too much fun and started backyard hunting when noone was home, or would visit my friend who had airguns too, a block over and shoot there. Then when I was 15 or so I 'd shoot the 760 Pump she had bought me (after I begged her) in the back yard when mom was inside, but if I shot a Starling, Grackle or squirrel, I didn't let her know about it. Just kept quiet and disposed of them in the 2-acre wooded area we had behind our back yard.
Eventually I shot a rabbit (I was maybe 16 at the most) and brought it inside. She wanted nothing to do with it so I went to my friend's house, skinned it, cut the head off and cleaned it with his father help showing us how to do it, and I brought it back. She liked it much better when it was all dressed and cleaned up and called my dad to ask him if he wanted rabbit for dinner. My dad loves all wild game meat so it all worked out fine. She or my dad still did not want me shooting any birds or squirrels but she did not mind when I brought home an occasional cottontail because she knew my dad liked it. I was lucky that I had a lake & farm land across the street and woods all around to go hunting on when I was younger before they built huge homes and wiped out all the woods later in the 90's. I was quite a rebel though and frequently went against what my parents asked me to do, but if you feel the overwhelming urge to shoot a Grackle or Starling, do it when noone is home and make sure they never find out. That means disposing of them properly and not leaving any feathers around or a pool of blood on the for evidence :-)
I hope you have Starlings too (black with yellow/green dots + yellow beak that congregate in large #'s) because you can explain to mom how most state wildlife agencies encourage their extermination since they are not indeginous to this country (Grackles are from here) and are a major threat to all song birds. Print some on-line articles and show her how destructive Starlings are. In some states officials spray chemicals to areas they congregate in to wipe them out.
I mean com'on guys it's not like he 's shooting someone's cat that trespassed, or doing drugs. It's just dang Grackles that harass and attack, sometimes kill song birds and deserve what they have coming to them. They are no different than Starlings in my book. If he disobey's mom's wishes it not like he commited a crime. The majority of the people don't like to see birds shot because they don't know all the facts.