It is natural to include animals in a system. They are beneficial in production of food, fertilizer , control of unwanted insects and plants and company. The overall food production should be more from a system which includes animals than without animals. Animals need to be selected for the desired end result and your system designed to cater for the animal. Animal care can be complex and some problems are too lengthy to be dealt with here.
Chickens: One of the most commonly selected animal. Advantages, provide high quality food in meat and eggs, provide high nitrogen fertilizer, can be used as cultivators and good for insect control in areas like orchards. Disadvantages, must be kept out of vegetable gardens as they are very destructive, can convert areas to dust bowls if their numbers are too high. Cultural, need protection from foxes and eagles, need to be kept warm and dry in cold weather but dont like temperatures over 30 deg. C, to lay eggs they need protein, need fresh water in dry weather. Common Health problems, chickens are generally easy to keep especially when they are kept in a free range situation, have access to dust bath and have a variety of food . Just dont accept any chickens, make sure they come from healthy stock, some diseases are difficult to eliminate once you have them. stickfast fleas very difficult to control once they have it. Consider killing the birds and restock after quarantine. Scaly legs is caused by a mite which lives under the leg scales, can be treated by scrubbing with warm soapy water then painting with vegetable oil or vaseline.
A disadvantage is that roosters are noisy. An advantage is that fowl can be put into mobile arcs to scratch over annual veg beds prior to planting. Another advantage is that fowl can turn over and break up weeds and spent vegetable plants prior to composting. They can also run quite effectively in an orchard (see Orchards as Ecosystems) They need to be provided with roosts and nest boxes.
Some info on stocking densities:
Below are a few notes on areas from various sources. Beware! They differ quite a bit.
I'd guess the 40m2 per bird is a good rule, maybe less if you rotate and let the ground recover or sow with a cover crop (e.g. buckwheat!) in between.
Some good poultry links:
Kim's Chicken Feed website
Pastured Poultry Resources
The Coop - Resources
Excellent publication on the potential for organic poultry production in the UK
The Duckweed Clearing House
Forsham Cottage Arks