Eggs. In theory I don't have a problem with consuming them. Chickens play an active role in soil fertility and pest management on a farm. They eat bugs. Yes, we feed our chickens grain; but given the opportunity, they will scratch around for insects to consume. This also aerates the soil. Plus, they are a ready source of fertilizer for crops! This taps into chickens' natural instincts, and as far as we can tell, yields happy poultry laying eggs. However, 79 billion eggs are sold in the U.S. every year, and the majority of those are produced under industrial conditions by only a handful of companies (horizontal integration). And thus my egg consumption stops.
This video from the Lexicon of Sustainability (which is awesome!), tries to tackle the often misleading terminology associated with eggs purchased in the grocery store. Free Range; Cage-Free; Humane; etc. While I would advocate going to a farmers market, chatting up your local hen-keeper, and finding someone you trust from whom to purchase eggs, I realize that won't work for everyone. So, at least be educated about your options, and read up on what the different labels mean for the treatment of the hens and sustainability of the egg-laying operation.
One last plug - just like pastured beef cattle, chickens that are allowed to "graze" on vegetative matter and insects likely produce eggs with higher levels of omega-3 fatty acids and certain vitamins. Now that's some food for thought!