I found an interesting article in Science Daily discussing the feeding habits of omnivores.  The study by researchers at the National Evolutionary Synthesis Center reveals that many omnivores descended from plant-eating animals or meat-eating animals, but not animals that ate both.  It is an interesting look at the change of diet over time.  I thought that when you discussed the transfer of energy within a food chain or food web it would be interesting to include this article.  It could provide a twist to the discussion that the students might find interesting.  www.sciencedaily.com/releases/2012/04/120416154417.htm