As one of the observers who saw a medium-sized dark-billed tern with a yellowish bill tip yesterday at Hayward Regional Shoreline, i want to lay out the timeline and my own analysis of the situation as i see it. On 6/15 at 2:40pm, Bob Richmond found, and then later both he and John Luther observed what they described as an adult Sandwich Tern in the main tern nesting pond at Hayward Regional Shoreline. Their observations are recorded in the eBird checklists below:
https://ebird.org/view/checklist/S57415267 https://ebird.org/view/checklist/S57413288
I can't comment on their sightings other than to note that they were likely significantly closer than we were for the majority of their observation time than i and the other observers the next day were. (They are both permitted to be inside an area where the general public is not allowed.) Also, i don't know of any photographs that either of them acquired on 6/15.

The next morning (6/16), Jerry Ting, Juli Chamberlin, Bob Dunn, Joel Herr, and myself spent several hours scoping the tern nesting pond from the Least Tern sign, the closest publicly accessible spot to view this pond. Jerry had arrived the earliest, and had already observed a few Elegant Terns along with one dark-billed tern with a yellowish tip that he obtained a distant, blurry flight shot of. All of those terns had settled on the far side of the NE island in the tern pond by the time the rest of us had arrived. This island is roughly 300-350 meters from where we were standing. Knowing the bird in question was likely on the back side of that island, we kept a close watch there.

While we waited, an Elegant Tern or two would occasionally pop up from the back side of the NE island, then settle back down, sometimes in view, sometimes out of view. About 70-80 minutes after Juli and i arrived, a medium-sized black-billed tern with a yellowish tip to the bill flew out from the back side of that island and landed on the front side, where it spent the next 15-20 minutes preening and getting us all very excited. Through our 85mm Swarovski ATX scope with 50x magnification and a 1.7x magnification extender, we could tell that the bill tip was distinctly yellowish, with the yellow restricted to approximately the last one inch of the bill. One oddity was that there seemed to be some pale coloration at the base of the bill, visible with scope magnification cranked up to the max (~85x). We knew that some juvenile Sandwich Terns show pale coloration beyond the tip, but in other respects the bird appeared to be an adult (and had been described as an adult bird by the two observers from 6/15). The forehead showed some white, which seemed to indicate it was starting to transition from alternate to basic plumage, though the black covered the crown and extended down to the nape. Wing color appeared very pale gray with darker gray wingtips, and legs were pure black. Crest did not appear quite as long and shaggy as Elegant Terns.

After approximately 15-20 minutes of preening on the front of that island, the bird in question flew. It circled around the back of the tern pond then turned and headed roughly in our direction toward the bay. While flying past us at a distance of no more than 20-40 meters, Jerry Ting managed to get a couple of flight shots of the bird, which can be seen in his eBird checklist here: https://ebird.org/view/checklist/S57431258
Those shots clearly show fairly extensive orange coloration at the base of both the upper and lower mandible. They also show the yellow coloration at the tip seeming to blend somewhat with the black coloration and extend further down the bill instead of ending in a crisp demarcation that would normally be expected on Sandwich Tern. The structure of the bill also seems longer and "droopy" at the tip, exactly like the structure of an Elegant Tern bill. None of this

was evident from our distant scope views, but i'm confident that the bird in the flight shots is the same one we observed preening on the front of the island. When i shared this photo with Alvaro Jaramillo, his opinion was that the photographed bird is an aberrant dark-billed but otherwise pure Elegant Tern, not a hybrid. Other expert opinions may differ, but it seems clear that either way, the bird we observed on 6/16 was not a pure Sandwich Tern.
This leaves open the question about the bird observed on 6/15. I see just two possibilities:
1)
There were two medium-sized terns that both have mostly black bills with a yellow tip, both present in the same area at
roughly the same time, associating loosely with a small flock of
Elegant Terns, one of which was seen by on
6/15 but (probably) not photographed, and one of which was seen and/or photographed on 6/16 by myself and others, yet at no time did any observers see these two dark-billed terns with yellowish bill tips together . 2)
There was just one tern with a yellow-tipped black-ish
bill, and the bird seen on 6/15 is the same tern that Jerry
photographed on 6/16.
Given the relative
rarity of dark-billed Elegant Terns in general, and the extreme rarity of Sandwich Terns in California, it seems to me unlikely that one of each
would show up at the same time in the same place, but never be observed together. In the absence of any other photographic evidence or further
sightings of the putative second bird, i think the most parsimonious
explanation is possibility #2 above. I was not present on 6/15, and the observers on that date were very likely much closer to the bird than we were, so it is entirely possible that the "two-bird theory" is true. I'll let the observers from 6/15 make their own case for their sighting. I've edited my own eBird report to dark-billed Elegant Tern for the bird i observed on 6/16.
Bob Toleno Hayward