Thanks to Richard Repp's eBird list comment, "juvie, could have been the model for big Sibley illustration" I realized it could be pretty to look at, especially in the afternoon light from the west side where we park. I emailed Mary, asking if she'd meet me there when she got done work, if I found it still there when I looked for it. She wrote back, "Sure". So off I went to pick up some supplies for my White-headed Woodpecker volunteer work, then park it at the Costco pond. The gull was still there. It was first reporting in the morning (by a lister who doesn't give credit to others in his eBird lists -- so no credit to him here). I saw Richard Repp there, having returned to look for a missing guidebook of his. Detective Columbo, as always. He'll solve it. The Bonaparte's Gull seemed to get agitated as the sprinkler service guy was walking around adjusting the heads, but when a large family unloaded from a huge SUV and plunked in the shade by the edge of the pond, Mallards came ashore and walked around them, while other Mallards puttered in the water close by, perhaps hoping for handouts. Eventually the Bonaparte's flew back from the far bank, and resumed its puttering right in front of us. It back & side paddled with its feet, sometimes lunging at a morsel it stirred up.
Oh, and the view. It was gorgeous. I've never seen so much detail in a Bonaparte's Gull. The tattered and faded feather tips against nice gray bases. Its fuzzy black earmuffs headset. Mary arrived in time to take in the view. It was still puttering when we left.