My test of whether a broody hen is worthy of eggs is that she spends two consecutive nights on the nest instead of sleeping in her normal spot. There are several things that indicate she might be thinking about going broody, Azygous described a few of them. I've had hens exhibit many of them for two weeks before either breaking them selves or finally switching to full broody mode but by not spending nights on the nest she indicates she is not ready to incubate eggs. That's about as simple as I can get. If she is not ready to keep the eggs warm at night, I'm not giving her eggs.
But my girls don't seem to be acting broody at all.
I bred my flock so the hens pretty much go broody every year once they are hens but they almost always wait until the hotter part of the year. You are too early in the year.
What breeds are yours? While any hen of any breed could possibly go broody, some breeds hardly ever go broody. Even with a breed known to be broody many hens will never go broody but your chances are much better if you have breeds known to go broody.
I'm wondering if they're stressed out by Cocky Balboa, the butthead Brahma cockerel... he is aggressive and awful to the hens, so I'm thinking maybe that's part of it?
No, it's not why they are not going broody.
What does that aggression look like? If it is to you or other humans I agree he needs to go. If it is toward the girls, could you describe it? It sounds like it could be normal behavior of an immature cockerel.
I trust you have another rooster to fertilize the eggs.
What else I should know about letting hens set eggs?
I do not start collecting eggs to hatch until I see a hen acting broody. Then I do the two consecutive night test to see if she deserves eggs. Once I've collected all the eggs I want her to hatch I start the all at the same time so I don't get one of those stressful staggered hatches.
Very important, make sure they all start at the same time.
When you get a broody hen you have two basic options. Some people isolate the hen when she is incubating the eggs, hatching the eggs, or when she raises the chicks. Some do all of those, some only do one or two. If you are going to isolate her during incubation I'd move her before giving her the eggs. One risk in moving a broody is that they break from being broody. If that is going to happen I'd want to know before I give her eggs. Sounds like you are planning that approach.
What I typically do is let the hen hatch with the rest of the flock. I mark all the eggs I want her to hatch and start them under her at the same time. Sometimes the other hens' eggs can wind up in the broody's nest so every day after the others have laid I check under the broody hen and remove any that don't belong. I think this method is easier when you have a larger coop. With the other method it doesn't matter how big the coop is.
Sounds like me when I get hormonal
That's funny but exactly. A broody hen is on hormones.
So when I see this behavior, should I leave that nesting box undisturbed and mark eggs? I've been collecting eggs daily, but obviously I'll have to leave the clutch that particular hen will hatch.
No you don't. You can give her the hatching eggs after she has proven to be broody. That's what I recommend so the eggs are fresh.
My hens go broody even though I collect eggs every day. I've tried letting real eggs (marked of course) build up and I've piled up fake eggs (golf balls) to try to get a hen to go broody. My goal was to get them to go broody earlier in the year. It never worked, even though I have hens that often go broody. They go broody when they want to, not when I want them to. But once you see a hen acting like she might go broody, mark about three eggs or use golf balls to help her stay interested until she flips over to full fledged broody.
Also, should I make a dedicated hatching box that's a little lower to the ground for the chicks to get out easier?
My hens hatch in nests 4 feet off of the coop floor and never have any problems getting their chicks safely down. After they hatch and she takes them to the coop floor they never go back to that nest. She takes them to sleep somewhere on the coop floor. The first couple of nights that can be in the middle of the floor but she soon learns to take them to a corner out of the way.
Since you are moving them, I would absolutely put that new nest on the floor. She will probably take them back there to sleep each night if it is still available.
What if more than one hen at a time goes broody? Should I just put both of them in there with their clutches of eggs and hope neither one breaks the other's?
I agree with Azygous, they are unpredictable. Some people have multiple broody hens on the same nest, they hatch the chicks together and raise the chicks together. Some have broody hens on separate nests in the same coop and have no problems. But I've had a hen on a different nest fight an earlier broody for the eggs when they start to hatch. They destroyed half the eggs, all with living chicks in them. Hens might fight over who controls the chicks, not willing to share. Some (actually many) people have said that they've seen one broody kill the chicks that hatch under the other hen. Since my experience I do not allow two broody hens to share a space. If you wish you can build two separate broody enclosures but I would not let them out in the same space to raise them. But like I said, others do that without problems.
What else? I killed a chick once by picking up a broody hen. The chicks, especially really young chicks, sometimes like to crawl up under a broody hen's wings. I picked a broody up one time to see what color her five chicks were. I crushed one that was under her wing. Since them I've learned to curb my curiosity. If I need to I'll still pick up a broody hen but I'm a lot more careful.
Some broody hens will tear your arm off if you stick it in the nest or threaten to pick up a chick. Some hens could care less if you stick a hand in the nest, no painful behavior at all. But most will get really excited if you pick up or threaten one of her babies, as she should.
If you do isolate that hen make sure the chicks cannot escape. If they get out and intermingle with the others where the broody cannot protect them they can be in danger. Make sure the wire you use has holes small enough the babies cannot get through, that it sets close enough to the ground they can't get under, and the gate or door fits tight enough a chick can't squeeze through. To me that one of the risks of isolating a broody, the chicks can get away from her protection.
One last thing. You cannot control when or even if a hen will ever go broody. The only way you have any control is to get an incubator. But, yeah, I really like my broody hens.