If chickens are fed with only treated grain, where do they obtain vitamin B12 from? Are these eggs a good source of vitamin B12?
In nature, poultry get their B12 by pecking for insects and worms in the dirt. Since this behaviour is not possible on industrial farms, B12 is added to animal feed1.
According to the USDA, one cooked egg (50g) contains 0.56µg of vitamin B122. The recommended daily intake of B12 for people age 14 and older who aren't pregnant or breastfeeding is 2.4µg3, though the FDA uses a higher RDI of 6µg for nutrition labeling4. At 9.3% of the FDA's RDI for B12, eggs do not meet the definition of a "Good Source" of B12, which requires 10-19% of RDI5.
When I personally consider what would be a "good source" of a particular nutrient, I am also interested in factors such as the foods other health effects, it's environmental impacts, and more. Wikipedia provides an overview of health concerns, and farming conditions for eggs.