sounds like a mink or weasel to me too. I have 17 hens, all heritage breeds, 12. 8 week old Americauna pullets, and 3 roosters, 1 dk brahma, 1 light brahma and a bearded buff laced polish...A weasel devastated my neighbors flock. They are like a mouse, if they can get their head in, it is all over. Bad part is, they kill for fun too. I pulled a 6 foot rat snake out of my coop, he went right up the ladder. when we cleared some brush. Just threw him back in the pasture...no harm done, no need to kill it. I have 1/4 and 1/2 hardware cloth ALL around the bottoms of my three coops/runs. Also concrete blocks or old ceramic tile buried makes a great deterrent. Good Luck!
I do have resident horned owl that lives in the pasture, and a few hawks. I have made it way too complicated to just fly in and grab a bird. They are great flyers,The way I have my3 coops, setup and lots of stuff hanging that they have great difficulty navigating. Plus my Brahma roosters weigh about 11 lbs each, and are about 2 foot tall...great guardians of my flock. BTW my smaller polish rooster is top rooster! lol