If something is wrong I'ld like to catch it early

[Bluepheonix13255]

I came out to my coop a few minutes ago andnoticed very dark somewhat runny poop- and somewhat more than I'ld expect from our 19 birds. I heard one poop and gave it a look: it was alittle foamy, but for all I know a little foam occasionally may be how chickens pass gas.
I give them some apple cider vinegar in all their water. Usually 1/2 a table spoon a gallon. If I am using some other water additive then I don't add vinegar, though I haven't used any recently.
I noticed Some poop on the outside of the latch- I assume that part of it might be finches or something getting grass (recently mowed) that managed to stick to our wire.
Even if it's not a medical concern, what may have caused this? I know next to nothing about chickens, and would appreciate some first grade instructions.
All the droppings are more or less as in the pick. None have anything particularly alarming.

 

[Eggcessive]

Chickens droppings can very with what they are eating, the time of day, and how much water they are drinking, especially in hot weather. The dark runny slimy dropping may be a cecal poop, which occurs a few times daily, and comes from the cecal pouches where food is fermented during digestion. Your vet may allow you to bring in a ziploc bag with some fresh droppings to check for worm eggs or coccidiosis. Droppings can contain mucus or blood, or be foamy, which are usually abnormal. Color can vary as well.