I think the crows are eating some kind of stickleback. This is an
annual event. I guess the sticklebacks die after spawning.

After the crows pick at the carcass, there is a bony almost
cylindrical section left from the middle of the body with the
eponymous spines on the back. You can find these on the docks down at
UW's Waterfront Activities Center, and if you find one with your bare
foot, you may need to put down the kayak you were carrying to pull the
carcass out of your skin. Guess how I know about this!

Mike