What is the most efficient way to get leather?
I'd have thought birds, since they breed quickly and can be kept safety underground, but I have trouble keeping them breeding. Even after banning eggs from my stockpiles and stopping the kitchen cooking eggs my birds really slow down breeding after a while.
I've read there is a maximum of 50 of each type of animal, so I keep them butchered to only have 1 male adult / 1 male child / 7 female adults / 7 female children of each species (using autobutcher from DFHack), but after a few years the eggs hatch very rarely. I'll occasionally clear out all the duff eggs (trade every egg in the fortress to a caravan), but that doesn't seem to help much (still a very low birthrate from the new eggs). Autobutcher automatically butchers the oldest adults and newest hatchlings, I'm assuming a bird can lay eggs the moment it becomes an adult? If it becomes an adult at 1 year old, and can't lay eggs till 2 years old, then it would explain my breeding problems.
If the 50 of each species is accurate, then would it be better to have a breeding set of each type of egg laying bird? Is there a maximum number of livestock before they stop reproducing?
I've currently got 4 species of egg layers (1 male adult / 1 male child / 7 female adults / 7 female children of each species), and plenty of wardogs, and only get maybe 10 hatching per year. I'm considering getting some pigs, since I've read they can be kept indoors, but if the reason for the slow breeding is the high amount of livestock, then pigs would make it worse.
For leather farming, is it better to just have 1 species or many? And are birds the safest choice? I keep my population cap about 30, so food is never a problem.