As I mentioned in my previous post, one of my targets for our trip to The Algarve was flight shots of first-calendar year gulls. This subject is covered quiet poorly in identification guides. Typically Lesser Black-backed Gulls appear slighter than most Yellow-legged Gulls, however, there is much overlap. The tail band of first year birds is usually diagnostic, but again there is variability. The tail band on most Yellow-legged Gulls tapers thinner on the outside edges. Lesser Black-backs generally have a broader tail band with less white on the upper tail and more black markings above the tail band. LBBGs don't usually show an obvious pale window on the inner primaries, but in bright sunshine both species showed some pale areas on the inner primaries. However, a pale window on the inner primaries is obvious on many YLGs although not a distinctive as on first-calendar year Herring Gulls.
Lesser Black-backed Gulls:
This last one shows a wide tail band, but note it has already replaced one of its central tail feathers. It bulky size large head and short stubby black bill all point towards Yellow-legged Gull.