Just wondering if this is a common food for a Great Blue. When I took this photo, I couldn't see what was in his beak.(thought it was some kind of worm, as most of the time only the tail was visible on my side) After processing, and lightening, I could see it was a seahorse. A bit of a shame, really, as I understand they are on the decline in the area I took the photo (Tampa Bay region). The GB kept dropping the seahorse, and then grabbing it again. Guess they are quite tough.
Great Blue Herons, and most other herons and egrets, are opportunists and will scarf down just about anything that resembles food and will fit down their very flexible necks. They've even been known to eat Black Rails driven out of the marsh by ultra high tides.
One of my favorite sightings was a Great Blue that had grabbed a fair sized eel right in the middle. Eels are incredibly slimy and slippery so it didn't want to let go, but the eel was using both its head and tail to beat him from both sides of his beak. I didn't stick around for the conclusion but if I had to place a bet, it wouldn't have been on the eel.
Paul: While photographing Great Blues at the Venice Rookery several years ago, one came in to feed the chicks something that was too much for the chick to swallow. It looked like a snake at the time, but after checking the images later, we determined that it was an eel.