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After noticing that River was a bit thinner than usual and that Simon was becoming a bit chubbier, it dawned on me to pay attention to where Simon was going when I first let him out of his cage after giving them breakfast.
I discovered that someone was being a sneaky little pellet thief and getting a second breakfast!
This forlorn look is his last-ditch attempt to guilt me into letting him have more pellets by acting as though he didn't just polish off an entire bowl of his own pellets and help himself to a half bowl of River's.  It's like he's saying, "Never mind the Timothy hay, human--that hardly counts as food!!!"
Happily, both of them are now back to their proper weight and Simon has developed a new appreciation for Timothy hay.