Dirty Vent

Hi P&C,
If they have been free-ranging in not so good conditions, maybe a good idea to worm them with flubenvent incase carrying a high worm load. Free-ranging is fab however with all those wonderful slugs and worms comes parasites particulary if the drinker stays in the same place for too long!
With their botties would be good to give them a good wash in warm soapy water(I use baby shampoo) and give a thorough check for any evidence of lice particulary around the vent. If necesaary a little clipping of feathers won't do any harm and will help stop the poo sticking to feathers.
Finally to help keep the gut in good condition pop a little unfettered apple cider vinegar in their drinking water, this is available at most feed or horsey stores. For more info on parasites and some piccies to help you have a lool at aunties sticky,

worms:
http://www.chat.allotment-garden.org/viewtopic.php?t=4348
external parasites:
http://www.chat.allotment-garden.org/viewtopic.php?t=5250

It probably means that they have eaten something at sometime that hasn't agreed with them and they've basically had the trots. Wash or cut away the surrounding feathers.