Dirty vent = runny bum.. ;D Most often caused by changes in diet or if your girls are not up to date with worming treatment.
Have your girls got more of or different kind of green stuff than usual? Lettuce is very common culprit for doing that and not recommended given other than small amounts now and then as treat ,it is not fibrous enough for their guts and run straight through them.
In spring when there is lot more new lush green growth about, chickens are like 'kid in the toffee shop', they love it all and it is not unusual them to have runny bum. In itself it is not causing them harm..but in long term it can lead to other health issues.
For now..keep eye on situation..keep their food in very basics..pellets/mash..little mixed corn for the night and fresh water and if you provide the greenery to eat..little amounts in few times a day is better than one big heap to scratch as they can eat themselves to death if getting to taste of it too much. If they are free ranging..well..cut back for any additional treats and hopefully it sort itself out.
Bottie bath every now and then is good and keep them wormed if that should be the case.
