To be truly accurate, I should have said that it’s the exit hole for reproductive (eggs), digestive (poop), and excretory (urine) products, but chickens obligingly combine their poop and pee, so there’s that.Ok. Well.... that will be a great four Hubby!
Aaaand today's chicken anatomy lesson has been brought to you by .... BYC!
It's ok - I appreciate your comments
I am looking at the picture posted above, is that your fencing? It looks like it has 2x4" openings, I don't know what it's called. I'd strongly encourage you to reinforce that with 1/2" hardware cloth like what I see on the right, at least to about 3' high. A raccoon can reach through the bigger openings and grab a bird and try to pull it through, with disastrous results. We lost a pullet that way through some field fencing several years ago. We also encourage people to lay an "apron" of hardware cloth on the ground around your run to a distance of 18 to 24" to prevent predators from digging under your fences. Ideally, wrap this up your fence at least a few inches.It's ok - I appreciate your comments
I watched a couple bathe today and kept reminding myself it was normal
Oh, we are going to reinforce it! The chickies are never out unattended either! And, yes, we've got plans to put some more underneath as b well!
Perfect!
I thought they were seizing. We do have one opportunist mischievous pullet that did peck at another, but the one she pecked at and her get along swimmingly and often gently peck at one another like 'Hey, you good?' 
They went limp sack of potatoes in my hand which only made me cry harder until the google page loaded with quite literally 'no, your chickens aren't dying - they're dust bathing.' 
I'm learning very quickly they're silly things, but it's a great way they help keep mites off of themselves. Sun bathing too, sometimes they'll sprawl out like something's killed them and won't even perk up until you pick them up. They're meant for Broadway!
Oh, my! Very happy your chicks are just drama queensFellow newbie to chicken keeping here. Don't worry, it's not just you! I panic googled 'what is wrong with my chicks?' when we first took ours out. A lot of my indoor chickies are 'dust bathing' early at under 4 weeks, but these guys never did so I near had a heart attack when we first took them out for a 'field trip.'
Hubby, who'd grown up with chickens, meanwhile was having the time of his life laughing so hard he couldn't even begin to try to explain to me that they were MORE than fine, while I was almost in tears furiously typing on my phone, hovering over the 'painfully dying' chicks who refused to get up because they were enjoying their dust baths so much.
Good Luck 
