Greetings
dylanwitt,
When feathers are missing from the vent and/or belly area, this can mean that the chicken is dust bathing excessively. They will grind their belly area so often that the feathers are buffed off. The vent area too, gets a lot of attention by the chicken. You may notice the chicken preening that area often.
It's not unusual for a chicken to dust bathe or preen, this is how they keep external parasites in check. But when they do it excessively (almost everyday), to the point of going bald, then, there is an issue.
Molting has specific patterns of feather loss. The entire feather will fall out, and new feather pins appear soon after.
But, feather damage is something different. You will not only see bald spots, you may also notice broken feather near the skin on the belly, and feathers that appear thin and in poor quality. Sometimes this can be due to poor nutrition, long term illness, or feather picking from other hens. But, if you see excessive dust bathing that is the clue that will tell you external parasites are most likely the cause. Sometimes, you may even see a chicken walking low to the ground or even take some steps backwards. Long term irritation on the skin can cause some weird behaviors.
What to do?
Take another look at the hen, this time, look for anything that looks like potting soil ground into the feathers. This is most likely lice and their debris (tiny, long shapes, dark colored bodies, looks like microscopic sticks) and dirt. You may even see egg clusters at the base of feathers. If it's mites you will seen them crawling around the chicken. Mites are tiny and round shaped. The hen may even have reddened inflamed skin on her belly and around the vent.
For treatment, if it's just one or two hens, you can give immediate relief by bathing the chicken in some dog shampoo. Then, dry her off with a towel and blow dry her feathers. I like to use Hartz Groomers Best Soothing Oatmeal shampoo for my hens, and for white chickens, Perfect Coat White Pearl Dog Shampoo Coconut. I just shampooed my white hen yesterday, and she was so happy and proud walking around showing off her fluffy feathers! She too, had a balding belly and vent area, broken feathers and poor feather quality. Or, you can just treat the chicken with something for external parasites. There are sprays and powders you can use.
But, it's Spring and, it really is time to start thinking, if not doing, a good cleaning of the coop. Then, treating it for external parasites, it's good to treat the flock too, since these pests do like to spread the joy. You know how it goes, first one gets it, then everybody has it. I use Elector PSP twice a year on the coop, run, and flock. No treatment will eliminate these pests forever, but, it will keep them from becoming an infestation of biblical proportions!
These are my thoughts on a possible reason for your hens missing feathers. I hope they are helpful.
God Bless and peace to you.