I have a problem with my dog. He's a cocker spaniel/ Alpine dachsbracke mix. So he's a bird hunter/scent tracker. The thing is that sometimes when someone drops something or he finds it laying on the floor, like socks, decorative objects, tissues... he will take it as a prey and won't let it go. He only does this with things he's not allowed to have, not with his toys. He does the same thing with mice as well. He will come show what he has, but when we tell him to drop it or try to take it away, he will growl and won't let go. He holds his "prey" very gently, doesn't kill it (the mouse in his mouth is still alive) and then if it's soft and small enough he will swallow it. Luckily he only swallowed tissues and paper until now. And maybe some mice. We always trick him with food or try to get him excited to get him drop it. This doesn't suffice, because we don't want him to growl at us and want him to release when asked.
Also, he's not a retriever. He doesn't bring his toys, he always wants us to run after him. But he will drop his toys without growling.
Anyone got any tips?