No thinly, heavily, or moderately veiled Bethbashing from me, I have said numerous times here I loved FO3. I am the Behemoth food source main complainer. Just because one thing in a game irritated me doesn't mean I have it in for the people that made it, just means I didn't like that part of the game.
I hated that NPCs in DA:O asked you to get out your credit card and buy some DLC, does that mean I hate BioWare.
Please don't attempt to twist my words completely out of context because I find it strange that a upwards of 20+ foot tall creature (which there are multiples of) in a fairly small area don't seriously mess with the food supply.
Bomfy -- sorry to have riled you. I guess we all have things about the Fallout games that annoy us even when we know better. It's the light switches that get me. Why, even in powered buildings, can't I turn on the lights? So, peace, sir, and accept my apology.
Anyway... if it helps, here's my take on the Behemoth food issue.
As far as what they eat is concernedt, it's clear that East Coast super mutants eat human beings. The gore bags are everywhere we find them. So there is an available food source, albeit one that fights back.
We don't really know how much they need to eat. You are right that in the real world a normal large animal generally has to eat a lot (the exceptions are reptiles, some of which can go for months without eating). Perhaps, in addition to food, they are partially sustained by the virus. Or maybe they are solar powered. The game doesn't tell us.
My theory, which I just made up, has to do with politics.
Now, we don't, as far as I know, possess much knowledge about the hierarchy of the Vault 87 mutants. None exhibit any great measure of intelligence; as Tabitha would say, they are dumb-dumbs. We could suppose that their power structure roughly corresponds to their physical might -- we'd end up with a progression from plain old super mutant up to overlord (despite their relative sizes I can't see a behemoth defeating an overlord, can you?). But one thing we are told is that the mutants keep growing as they age (I don't have a reference for this. Does anyone remember where this is said?). The conclusion one could draw from this is that behemoths are very old overlords, very huge but somewhat enfeebled with age.
Is it possible that those venerable old mutants, so few in number, are being fed, as tribute, by their lesser brethren? Are those trinkets we find in the bags really there by accident, or are they part of the tribute? We know behemoths carry a lot of stuff they don't use.
Or, like that guy noted above, raiders. You have to admit, they breed like rabbits out there. And, apparently, keep their kids in holes.
Just like rabbits.
Please don't get me wrong. I'm not claiming that in-game data supports this. I'm just trying to use the available lore to explain an in-game mystery. It's something I find myself doing way too much.
(For example, did you know that robots are refilling the soda machines? And that food preservatives in the Fallout world were so effective that treated food
never spoils? And that stimpaks and other drugs go bad unless they are kept in special, high tech first aid boxes?)