so basically I got my chickens about a week and a half ago and we never had problems with them getting attacked, but..... today my cousin's dog (their our neighbors) visited us and at first, it was pleasant and we petted him and everything was fine, but then he went sniffing around the fence of the run, and I wasn't stressed at all because he had done it before. so I continued to pet my dog for about 30-ish seconds and then I looked again to check on the dog and the chickens, but this time the dog wasn't just sniffing around, he was on the wall that the fence is attached to (keep in mind that I tried to barricade it as much as I can) so, at first glance, it wasn't anything serious at all, because he is a very but I mean VERY dumb dog (and I'm not exaggerating at all) so he somehow finds the tiniest hole (he's a small dog) between the fence and a tall bush that (supposedly) keeps predators from coming in, also that hole is covered by leaves so I didn't think he will notice, but then. I noticed that he was tracking one of my chickens through that hole, so as I was getting up to move away, he just JUMPS in there and starts chasing my chickens, keep in mind they're only 4 months old so they can't even try to defend themselves so I just start freaking out and without even thinking twice I help my cousin get in there and pick up his dog and teach him a lesson, but... he had already caught one when my cousin picked him up so my cousin started hitting him so he would let go of the goddamn chicken, but he couldn't so I opened the dog's mouth with force and successfully released the chicken, but while I did that I've realized it was too late I've already heard bones crunching and she was dead.. so yeah I know its a long story and I have some mixed feelings about it, but I have to focus on the important thing which is how to prevent that from happening in the future, so please reply if you feel like it because I care about them. here are some pictures of everything
(the last picture is a picture of the hole he jumped through)