You may have noticed in the last year or so that you have more choices on the egg shelves. I'm not talking about more egg substitutes, although they probably exist as well. I'm talking about the influx of eggs sold as omega-3 eggs. That is, eggs enriched with higher levels of the omega-3 polyunsaturated fatty acid. The farms that breed chickens for these eggs replace 10-20% of the chicken feed with ground flax seed, a natural source of omega-3. By replacing 10% of the feed with ground flax seed, the amount of omega-3 contained in one egg can be as much as 10x the amount contained in a classic egg.
Carmichael Training Systems - Essentials for Nutrition January 2007
Canadian Egg Marketing Agency