It depends on the chicken: how big it is, whether the chicken is growing or molting or laying eggs, what the weather is like, and how efficiently that particular chicken uses food.How much food do chickens need per day?
Do they eat it all, or do they stop at some point?It seems like the popular answer is free choice but we've been having a rat problem so I have to remove their food as soon as they finish eating. I feed the 5 of them about 1-1.5 cup 3x a day.
Are you feeding it dry, wet, or fermented?
I would start by letting them all eat until they stop, no matter how much that takes, at every feeding. After a few days, you should have a fairly good idea of how much they need, although the amount will still change from time to time (growth, egg laying or not, cold weather or hot, etc.)
I found a website that says chickens need about 1/2 cup of feed per day.
I've seen lots of websites that estimate 1/4 to 1/3 pound of dry feed per laying hen per day. I found one place where someone measured, and found that was about 3/4 cup to 1 cup (which might be different for different brands of feed.)
Weighing or measuring feed when it is wet means you are weighing or measuring the water too. Water is heavy, and feed swells a lot when wet, so the measurement is very different even though the amount of actual feed has not changed.
If "overeating" means eating so much it is bad for them, they are probably not.I used to only have 2 and maybe they did eat about that much per day but now that we have 3 extra chickens, it seems like my two earlier birds are fighting for the feed and maybe even overeating.
If your chickens used to eat both fermented feed and dry feed, and you took away one kind (because of the rats), of course they will be eating more of the kind they still get.
It would help if they can all eat at once.It's tough for me to gauge how much to leave out especially since they pick on the smallest one and kick her away and I'll see her pecking at the empty bowls later. I pour out more food for her and the other ones that I thought were full will come back and keep eating! I feel like I'm overfeeding them but I also want to make sure nobody's hungry.
That might require more bowls, or maybe a long trough (some people use plastic gutter, like is used on a house to catch rain.)
It might also help if you have a way to keep them from seeing (or noticing) the smallest hen so much as they eat. Maybe you could put bowls in several different corners, so their heads face in different directions as they eat.
Chickens like to eat small amounts all day long, so it's hard to tell whether picking at the empty bowls means she needs more food or not.
You could feel the crop of the small one, or all of them, after they finish eating. A full crop is fine. An enormous bulging crop is probably fine. An empty crop, or one that is almost empty, could indicate a bird that did not get enough (especially if it is the one that gets chased away.)