Belcher's? What Belcher's?
Kelp Gull. Young Kelps, of course, have black on their tails. Euclides' photo of the bird shows it to be the same size as the adult Kelp Gull next to it, and that would not be right, as the average Belcher's is smaller than a Lesser Black-backed, and the average Kelp is larger than a Herring.
That made us look at the Jan Axel photos more closely (and not just out of the envy of never seeing the bird after repeated tries). The bill looks too heavy and blunt for Belcher's, which would look "sharper", but just right for Kelp Gull.
The final nail in this coffin is laid by Bob Ridgely:
To me this looks like a subadult Kelp Gull, and not a Belcher’s (Band-tailed). Head shape and overall jizz is off, as is the leg color; Belcher’s typically has bright yellow legs when adult, which by plumage it would have to be. Also, no one would ever consider Belcher’s a “huge” gull. But I’ll admit that on occasion I’ve had to look twice at solitary birds in Peru; size can be subjective.