A very uneventful Sunday in the park, with little more to see than the Black-Headed Gulls at their usual game of knocking each other off their perches.

There must be some element of status here: high-ranking gulls can expel low-ranking ones, but sometimes you see a gull vainly flying up a line of posts with other gulls perches on them, and failing to displace a single bird.

Then on the Serpentine I came to a mob of large gulls raiding the food some people were throwing for the swans. The usual large gulls on the Serpentine are Lesser Black-Backed Gulls and Herring Gulls: easy to tell apart when they are adult, because the former have dark grey backs and yellowish legs, and the latter light grey backs and pinkish-grey legs.

These big gulls take four years to grow up (the medium-sized Common Gulls take three, the smaller Black-Headed Gulls two). From their second year onward they have distinctively dark or light grey adult feathers gradually replacing their tweedy brown juvenile plumage. But when they are in their first year, they are the same colour, and this makes them hard to distinguish. Here are two first-years: which is which?

There is some variation in plumage colour, but this applies to both species. On the whole, Lesser Black-Backed Gulls have more black on their beaks than Herring Gulls, which tend to have black tips or bands on their beaks, but this is not invariable either. Herring Gulls tend to have flatter heads than Lesser Black-Backs, but this can be hard to spot in practice and there is a lot of overlap in shape.

The most reliable clue is the thickness and shape of the beak. A Lesser Black-Back has a fairly slim bill, and the slight bump on the bottom mandible a third of the way from the tip (known as the gonydeal angle) is not very prominent. A Herring Gull has a thicker bill with a more pronounced bump underneath. So the first of these young birds is a Herring Gull, and the second is a Lesser Black-Backed Gull. (And even after writing this, I'm keeping my fingers crossed in case I'm wrong.)