Tuesday, 16 August 2011

Identifying juvenile Yellow-legged Gulls

By Keith Vinicombe 

Juvenile Yellow-legged Gull (Rainham landfill, Greater London,
16 August 2011). Photo by Dominic Mitchell.

Identifying immature large gulls is not everybody’s cup of tea, but there is an annual influx of Yellow-legged Gulls Larus michahellis in July, mainly into southern Britain. The species was finally split from the familiar Herring Gull L argentatus in 2006, although most birders probably had it on their lists for a couple of decades before this!

Yellow-legged Gull is a common and familiar sight in the Mediterranean, and these large gulls have spread up the west coast of France and the odd pair now even breeds in southern England. If seen well, adults are readily separable from Herring Gulls by their obviously darker grey mantle and yellow legs. Before tackling juveniles, it is advisable to get to know the adults, so that you gain a good ‘feel’ for the species.

The Yellow-legged Gulls that arrive here in July are usually large, heavy and sturdy birds, with a large bill with a distinct gonydeal angle on the lower mandible. The males in particular can be very large and often approach Great Black-backed Gull in size and overall bulk. In fact, a few days ago, I actually watched one persistently displaying to a Great Black-backed! When scavenging on dead fish – a favourite food source – they are very much second in the pecking order behind Great Black-backed and dominant over both Herring and Lesser Black-backed Gulls.

Like Great Black-backed Gull, large male Yellow-leggeds appear heavy and ponderous in flight. Females, however, are smaller, as are Yellow-legged Gulls from western Iberia, which appear in south-western England later in the autumn.

In flight, Yellow-legged Gull has very long and pointed wings compared with Herring and, on inland reservoirs, they can often be picked out at some distance – even with the naked eye – as they patrol up and down on arched wings, about 30-50 m above the water’s surface, looking for fish.

Juvenile Yellow-legged Gull
Juveniles are much more difficult to identify than the adults, but the following photographs should shed some light on their identification. Simply look at the images and soak in the plumage and structural differences.

Juvenile Yellow-legged Gull (Chew Valley Lake, Somerset, 21 July 2011).
Photo by Keith Vinicombe.
Plumage-wise, the following are the main points to note:
• Yellow-legged is more similar to Herring than Lesser Black-backed in overall tone, being brown with whitish feather fringes.
• Note the very white background colour to the head, neck and breast.
• Note also the grey ‘mask’ extending back through the eye. This can be very obvious on some individuals and, when combined with the white head and neck, it can create an impression distinctly reminiscent of a juvenile Mediterranean Gull.
• The entire upperparts are mid-brown, with clear-cut whitish feather edges. In particular, there is little chequering compared with juvenile Herring Gull (see photo below).
• Note the plain brown tertials, with a neat white fringe to all the feathers. The tertials are much more similar to those of juvenile Lesser Black-backed Gull than juvenile Herring, which shows heavy chequering around the edges. Note, however, that some juvenile Yellow-leggeds do show pale ‘notching’ or chequering towards the tips of the tertials.
• The greater coverts – the band of large feathers that runs along the bottom of the closed wing – have plain dark bases, particularly towards the front of the bird, this forming a dark bar. Two points to be aware of: firstly, it should be noted that the greater coverts are often hidden by the fluffed-up flank feathers; and secondly, many juvenile Yellow-leggeds show quite noticeable chequering on the inner greater coverts (those towards the tail end of the bird).

In flight, three features are worth remembering: first, the pale ‘window’ on the inner primaries is less obvious than on Herring, but much more obvious than on Lesser Black-backed; second, the upperwing coverts contrast quite strongly with the black secondaries and there is often a ‘second’ darker bar along the outer greater coverts, in front of the secondary bar; and third, the underwing coverts are dark brown, more like Lesser Black-backed Gull than Herring. The following photograph of a first-winter Yellow-legged Gull (taken in March) illustrates these points.

Yellow-legged Gull (Chew Valley Lake, Somerset, 30 March 2011).
Photo by Keith Vinicombe.

It is also worth remembering that, being a more southerly species, Yellow-legged Gulls breed earlier than Herring and Lesser Black-backed Gulls. Consequently, being more advanced, they moult earlier at all ages. The settled bird shown in the first image above (photographed on 21 July) has already acquired a few darker and greyer first-winter scapulars.

Juvenile Herring Gull
The juvenile Herring Gull below (photographed in September) is similar in plumage tone to Yellow-legged Gull, but note in particular the heavily chequered wing coverts and the chequered fringes to the tertials. Also, it lacks the dark bar across the outer greater coverts, shown by Yellow-legged.

Herring Gull (Praa Sands, Cornwall, 15 September 2009).
Photo by Keith Vinicombe.

Herring Gull is much more heavily and uniformly streaked across the head, lacking the striking white background colour and the dark mask of Yellow-legged. In addition, it is a slightly smaller-billed and more compact bird, its shorter primaries producing a somewhat truncated rear end compared with the long-winged Yellow-legged Gull. In flight, it is a rather stocky, relatively short-winged gull with noticeable pale inner primary ‘windows’.

Lesser Black-backed Gull
Young juvenile Lesser Black-backed Gulls are dark and rather smoky looking compared with both Herring and Yellow-legged. They are densely streaked and the feather fringes are darker and buffer, contributing to the overall darkness and uniformity of their plumage.

Lesser Black-backed Gull (Chew Valley Lake, Somerset, 29 July 2011).
Photo by Keith Vinicombe.

Note in particular that, like Yellow-legged Gull, Lesser Black-backed has plain tertials with a clear-cut pale fringe (if present, any ‘notching’ is slight and confined largely to the tip). Like Yellow-legged, Lesser Black-backed also tends to show a dark bar across the outer greater coverts.

On average, Lesser Black-backed Gull is the smallest of the three species, with a shorter, weaker bill and a long, slim and rather attenuated appearance; the latter is caused by the long primaries – a consequence of the fact that, unlike Herring Gull, Lesser Black-backed is a long-range migrant, with most heading down to western Iberia and north-west Africa for the winter.

In flight, Lesser Black-backed Gull has long, slim wings – producing rather a slim, narrow-winged, rakish impression – and juveniles appear particularly dark and smoky. Juvenile Lesser Black-backed is, in fact, much easier to identify in flight than at rest. Note in particular the very dark underwings (with only a faint pale window) and very uniform upperwings. Most importantly, the black of the upper secondaries extends right across the inner primaries to the outer primaries, forming a completely dark rear to the upperwing. Juvenile Lessers are also darker and more heavily marked on the rump.

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