Mammals make milk and birds make eggs for extra product beyond meat. Milking small mammals can be impractical. Mammals can provide fur and birds can provide feathers. Feathers need another form of cloth to hold them in place but fur and leather are functional by themselves. Few birds are grass eaters but many domestic mammals are (and grass is by far the most efficient product of agriculture in terms of food per area and in terms of fuel/fertilizer/pesticide/man-hours use). Mammals are less likely to harm each other due to boredom like chickens do. Birds are easier to control reproduction by removing eggs or even adding fake eggs. I am posting this to decide if I should stick to the standard chickens for a small scale animal raising operation or pursue my idea of acquiring some South American rodents that have been domesticated for centuries as meat animals (a large variety of guinea pig called "cuy" -pronounced "coo-wee" and a rabbit-like species of vischasha without the face markings of the wild species). I read somewhere that small variety rabbits grow twice as efficient as chickens in terms of calories per dressed meat weight and of course most of those calories are grass for the rabbits. I raised a baby hare indoors and gathered her poo and mostly fed her rabbit pellet food. Her food volume was about twice what her poo volume as she doubled her weight every few days (10 times as much over her first 2 weeks). I am going to have to find official numbers for several species of both birds and mammals (and maybe fish too) and see if I can find relationships that might help me guess what other species might be more effective choices. Goats have unusually large teets for their size and I suspect that might have moved them into one of the more popular meat animal around the world. Rabbits have great furs but being rather cute, I think they have negated their advantage as a meat animal. The animal I was most emotionally bound too was my second 4H steer with my first steer also scoring high on my most loved animals. This puts me in the perspective to see all species equally suited as meat animals as I know they are all smart and emotionally sensitive creatures. I even had some fun relationships with a wide variety of bird species including several chicken varieties so their brains take no position in my choice unlike a lot of people who point-blank declare their meat species choice to be stupid or emotionless. Pigeons and doves have a homing instinct and poor night vision making them a meat animal that can find their own food in the wild. Keeping mammals in the wild usually means they must be big enough to deal with wild carnivores (cattle, llamas) but I think ground-dwelling animals might also be effective at keeping safe from predators (European/domestic rabbits, vishachas). Emus are a bird raised ranch-style for meat as they can out-run any predator, flightless and too big to go through sheep fencing. Personally I could go vegetarian for "green" reasons but people love eating meat and we are definitely built to be omnivores instead of the pure herbivores like the rodents I am currently raising (for pets), degus, chinchillas, and lemmings. I can see lots of specializations they have for being pure herbivores that humans lack. Thus to teach the world how to be food-independent, meat-raising methods need to be studied and taught too (and maybe invented such as a new species or new methods or new byproducts from the live animals). Fish, reptiles, amphibians and most insects would not be functional outdoors in colder climates and other factors limit their potential but I am open to experiment especially if I am raising carnivores (as fur bearing animals likely but pets too or even as lower levels on a food chain such as fish farming). Microscopic animals like krill diatoms and amoebas are also possibilities but again, not likely acceptable for human consumption.