For those looking for a small animal to keep that produces a high powered fertilizer I say take a look at pigeons. Their manure is a horticulturalists dream manure. It is high in nitrogen, potasium and phospherous. It also sells at a higher price than any other manure out there. Pop it in your
compost (however you compost) and it will work wonders for you.
Each adult pigeon of normal sized breeds eats about an ounce of grain a day (though the Utility Breeds will eat more, being about the size of a bantaam chicken). Pigeons raise their own young, often with two different nests growing up at a time. Most of the care you must do as their keeper is feeding twice a day, changing water and setting out some bath water every other day for a short period, then clean out their housing of the manure once a week. The pigeons will do all the work of rearing their young and the squabs grow super fast. Female pigeon lays two eggs and 14-18 days later they hatch. A squab goes from hatch to fledgling in about 28 days. At 25-28 days the squab can be harvested. This is the ideal time to harvest as the flavor goes very gamey after they are fully fledged. About 10 days after the first set of eggs is hatched the female lays two more in another nest while the male keeps busy feeding the other nest. While most of their squabs are produced in spring and summer, they will keep going all winter provided they have enough food to eat. Most pigeons will produce squabs that are around 8 ounces after processing. Utility pigeons produce squabs that end up around 12 ounces and are the most prolific breeders of all pigeon breeds.
Obviously they won't replace chickens or ducks as egg producers, nor produce as much meat. They are however a constant source of meat for a small family with a good price point for feed. Many other small livestock are much longer of a wait to harvest, and you end up harvesting all at once and freezing most. Once your loft is in production mode you will be harvesting 2 birds per every other nest every month of the year. Often you will end up harvesting every week as your birds will be spread out on egg laying times.
For smaller squabs most people can eat 1 1/2 to 2 per meal but the Utility breeds one per person is plenty. They are also just incredibly tasty. They are a dark meat which has a definite berry like flavor. The adults can be eaten as well, but tend to be gamey. Pigeon pie is about the only way the adults are worth eating. Best results will be harvesting squabs since the adults are so good at raising them quick.