This is from a article I read...
These tiny eggs may or may not have a yolk. Yolk-less fart eggs are often called rooster eggs. These little eggs are often much darker than normal, as they spend more time in the shell gland pouch and gets an extra layer or two of pigment. These yolk-less eggs sometimes form when:
- The hen's oviduct releases a small piece of reproductive tissue or another small foreign mass enters the hen's oviduct, triggering the regular formation of an egg. The foreign object will be treated like a normal yolk and enveloped in albumen, membranes and a shell;
- Occasionally a hen will also lay a fart egg when something disturbs her reproductive cycle;
- Young pullets may lay a fart egg or two when coming into lay and is still getting their reproductive systems in gear.