Well, I figured out what the heron was eating...
 

....from the stream behind my house. Frogs. Lots and lots of frogs.
Plus crayfish. I wandered around in some larger pools and could find
any crayfish larger than an 1" long. They try to pinch, but it doesn't
do much good with such tiny claws.

I guess I'll have to stake out a well-hidden shooting position to catch
him in the act next time he's in town. I'd rent a 70-200mm f/2.8 L IS
for a day or two if I knew he was coming every day like he was earlier
in the summer, but I think he cleaned out the stream pretty good and has
moved to greener pastures.
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