There were numerous juveniles such as this on our visit. The distinct white primary tips and large mirror on the underside of p10 are features of C. m. glaucodes according to Howell & Dunn, but this race is not generally recognized. This wing-tip pattern also helps distinguish juveniles from Black-headed Gull (Chroicocephalus ridibundus) which has less obvious white primary tips. The narrower dark tail band of Brown-hooded is another supposed feature but the difference is not evident to me in this photo.