My friend Julia is concerned with animal welfare. She seems to have been the first in the world to come up with this obvious-in-hindsight metric for evaluating animal calorie sources: how many food calories does one animal’s life produce?
The differences turn out to be many orders of magnitude. There are vegetarians who eat eggs but avoid beef, but because cows produce many more calories than chickens do, this is just trading twenty chicken lives for one cow life. Maybe a cow’s life is actually worth twenty chicken lives, if cows are more complex or emotional or whatever the morally-relevant quality is. But this isn’t clear, and I doubt most vegetarians are aware of the trade-off.