Not ALL Humane Societies are "no kill" by the way! (Ask me how I know!) I've had to do SEVERAL drop offs (my neighbor went through a phase of taking in EVERY stray dog and then dumping them once she got 'tired' of taking them outside in the cold weather).
I ended up with cat #4 (stray) cause our Humane Society told me they'd IMMEDIATELY euthanize her if she was confirmed to be pregnant (she looked it).
Most of them will have some kind of after hours drop off kennels. SOME will even have video monitoring on said kennels (maybe people trying to steal strays for fighting, medical experiments etc).
Since the owners will more than likely NOT change their behaviors...I would simply turn the animals in, notifying the animal control/pound people that you found these dogs on your property, killing your chickens. If they want your name etc...give it to them. IF your neighbors DO find them at the pound (UNLIKELY they'd go looking for them)...the pound people probably will FINE them AND could use your information to justify the fine or even collect penalties for the destruction of your birds.
One dog I had taken for my neighbor (really SWEET pitbull), was an awesome sweetie. I put him in the after hours kennel and left my info. They DID call...but I believe that was after his owner had picked him up. They were just verifying WHERE he was found to determine if the owner WAS his I'm guessing. (He was from the neighborhood)
Either way...good luck, make whatever decision YOU are comfortable with (or have your hubby take care of it so you really DO have ignorance).
Let us know what you decided and GOOD LUCK and congratulations on your sister's baby! Travel safely!