Is a chicken going to eat their caloric (energy) need OR a volume/weight?
Seems like a simple thing to me... If they eat to meet their energy, the calculation are based on calorie... and then wrapped into a tidy package that gives us a "weight" guideline for feeding... the must keep it as simple as possible for consumers. My dog feed always has kcal content listed.... and yes they do list their MAX moisture content also as a % of weight in the "dry" feed (since I presume moisture cannot be listed as a % of calories), in addition to fiber usually.
If I pack all fat calories which will be really dense... will my chickens then eat the same volume making for a much higher calorie content or would they simply stop once their ENERGY need is met?
Is the protein in our eggs meeting an energy need or a volume need?
I don't think it matters as long as you are consistent.
Typically, each organism needs:
1) A certain number of calories per day that depends on the individual but will usually fit in a small range particular to that species, its sex, age, reproductive status, activity level, etc.
2) A certain number of grams of protein per day, also depending on the individual but fitting into a narrow range particular to that species, its sex, age, reproductive status, activity level, etc.
3) A certain number of grams of fat, carbohydrate, vitamins, minerals, etc. that depend on the individual but will usually fit in a small range particular to that species, its sex, age, reproductive status, activity level, etc.
Since all the nutrient are in narrow ranges it is typically easiest to list them as percentages and since feeds are made with recipes it is easiest to list them as weight, not calories.
As for what chickens are eating - if your chickens are getting fat, something that is apparently an issue in backyard flocks, they are eating more than their caloric needs. If they are losing weight they are not eating enough calories. For animals (even humans) that need to loose weight a typical tactic is to replace a portion of the daily intake with an equal volume of a less calorie dense food so that they feel "full". Think giving green beans to dogs that need to lose weight.