Get eggs from chickens (put a hopper under a chicken spawner and wait awhile, they'll produce eggs).
Then use the eggs to spawn some chickens. If you have more than 5 chickens in the same area, they'll despawn, so you'll probably want to make several small enclosures for your chickens and spam eggs until each has at least 2 chickens. I'm not sure how far apart they have to be.
Then feed those chickens wheat seeds. They'll reproduce. Some of them will despawn once you get over the 5 chickens in an area limit.