You have to consider what it is the beak dogs are eating in such quantities to be able to lay all the eggs and provide all the meat that the goblins get from them. Food production and consumption in DF isn't realistic but should improve over time.
In terms of both realism and game balance, there are a lot of aspects of DF that could use tweaks, not just food production and consumption. If you question what beak dogs could eat in order to (theoretically) provide enough eggs and meat for goblins, consider: (1) There are many carnivores in the game and,
unlike grazers, what they eat is not simulated. This includes even common domestic animals like dogs and pigs. (2) I'd imagine that Beak Dogs would be much like pigs from hell: They'd eat
practically anything, from roots and whole plants to bugs and other vermin, from tripe and other animal/humanoid parts to feces. (Pigs really are
that gross!) (3) Goblins would most likely not rely entirely on Beak Dog eggs and flesh, also consuming other things.
Current farming (especially underground) throws up more believability issues for me than goblins, too.
While it may seem far-fetched for such to grow fast enough to sustain a civilization, underground farming does not give me much pause in terms of believably. Life has a way of filling empty niches.
Bacteria can grow not just in soil, water, and flesh, but also in acidic hot springs, radioactive waste, and in rocks nearly 2 miles deep into the earth. They can even subsist on dissolved compounds, like hydrogen sulfide and methane near hydrothermal vents. Some microbes
can even grow in the vacuum of space.
Aside from bacteria, certain types of
fungi thrive in the absence of light.
Mushrooms and
molds are able to derive nourishment from breaking down organic matter.
As for what organic matter sustains underground crops in DF, they grow on soil or muddied rock. Many soils are rich in organic matter. We can also use fertilizer to increase yields. And while not simulated, what do you imagine happens to the feces of the inhabitants of a fort, particularly the animals and any imprisoned creatures?
Myself, I like to imagine that many underground crops are similar to
lichens. Lichens are not plants. They're two species in a symbiotic relationship - either fungus with algae or fungus with cyanobacteria. Now, imagine an exotic fungus in a symbiotic relationship with a bacteria that can break down the minerals in clay or soil to form nutrients it can use.
If that
still sounds too fantastic, consider:
- Nitrogen fixation is "essential for all forms of life". Certain plants, including legumes have a symbiotic relationship with prokaryotes (bacteria and archaea) for the nitrogen they need. Even a few animals (termites) have such a symbiosis.
- "...over 90% of all plant species engage in mycorrhizal [symbiotic] relationships with fungi and are dependent upon this relationship for survival." (Reference)
- "Most bacteria have not been characterized, and only about half of the bacterial phyla have species that can be grown in the laboratory." (Reference)
- Bacteria have been discovered that can break down plastics and oil spills.
- endoliths: An "organism (archaeum, bacterium, fungus, lichen, algae or amoeba) that lives inside rock, coral, animal shells, or in the pores between mineral grains of a rock."
- Prevailing scientific thought is that complex life could only evolve after acquiring mitochondria. The endosymbiotic hypothesis suggests that this happened, "when a cell engulfed some bacteria and started using them as power generators".