I spent much of my December birding time watching gulls as they drifted along the cliffs in Hastings. There seems to have been a near constant stream moving west here and the process of scouring them for rare species is quite addictive. The first clue that something was happening came in the first week of December, when a number of Caspian Gulls were reported on Sussex birding networks. Back from a trip away and turning my attention to West Hill, I noticed groups of large gulls, including many Lesser Black-backed Gulls (generally quite scarce here), passing below Castle Rocks.

Over the coming weeks I spent a few early morning sessions on West Hill watching and photographing passing birds. Among the three regularly occurring species, I saw nine unique Yellow-legged Gulls and six unique Caspian Gulls, a surprising total considering the relative scarcity of these species locally. By the end of the month numbers had slowed to a trickle. In this post I’ll talk about the birds I saw and their flight identification and then at the end there’s a bit of analysis of species composition. Hope you enjoy grainy low-light flight shots of gulls btw 😉

Caspian Gulls

Species	Total	1CY	2CY	3CY	4CY+
Caspian Gull	6	3	1	1	1

In flight, immature Caspian Gulls stand out suprisingly well amongst other large gulls. The first visual clue is the clean white head with streaked boa around the neck, at which point I check for the structural features like the head/bill shape and over plumage impression. I’ll then grab a few photos so I can check the plumage details later. Adults are a bit harder, more on that below.

1CY

The most common age class, I noticed that 1CY birds tend to have a more contrasty impression in flight than same age Herring Gulls, almost like Great Black-backed Gull at times. The venetian blind pattern on the inner primaries, greyish saddle created by the 2nd generation scapulars and tail pattern all stand out in flight which helps with getting onto a candidate bird. I find the dark greater covert pattern is usually only possible to pick out in photos at this distance.

Below are some typical low-light views of a 1CY Casp moving along the seafront.

The bird below was in slightly better light. This was a large individual and when I first picked it up in bins I thought it was a Great Black-back.

2CY

Views were fleeting of the one 2CY bird I saw. Its slightly more contrasty plumage and white head made it stand out against the Herrings it was travelling with. The plumage in this age class is more advanced compared to a 2CY Herring Gull with much adult-like grey in the mantle and wing coverts and a white mirror on P10. The more likely trap is 3CY Herring but this species would show a different primary pattern with black only on the outer primaries and adult-like grey inner primaries.

3CY

For the 3CY bird I saw the standout features were once again the white head and neck shawl, here combined with bold black markings on the outer primaries and primary coverts. The main confusion species would seem to be Yellow-legged Gull, which can be eliminated here on head/bill characteristics, mantle colour and primary pattern. Mantle colour wasn’t very useful in this instance due to the strong sunlight but the head/bill stood out enough for me to get onto it. In the photos the primary pattern shows solid black band on P5 and black spot on P4, long grey tongues cutting into P7 and P6, and white mirrors on both P10 and P9.

4CY+

Adults are the trickiest to pick out for me. Even in December there seem to be lots of white-headed adult Herring Gulls moving through and assessing the crucial primary pattern within the few seconds you have is pretty difficult. I picked this one up on hue of the grey tone, the plain and slightly darker grey standing out after watching hundreds of more blueish grey Herring Gulls fly past. Structure and primary pattern were checked using photos. Maybe not a perfect example, with a bit too much black on the outer primaries and some expansion at the gonys in the bill, but good enough for me.

Yellow-legged Gulls

Species	Total	1CY	2CY	3CY	4CY+
Yellow-legged Gull	9	1	1	3	4

The Yellow-legged Gulls I picked up were mostly older birds so not sure where all the 1CYs are at the moment. The dark bluish grey mantle tone of these older birds really stands out after you’ve watched hundreds of Herring and Lesser Black-backed Gulls pass by.

1CY

This bird was not actually passing through but rather hanging around the seafront for much of the month. It seems to have a damaged wing, although it can still fly. Hopefully it heals up and moves on. Looks pretty classic overall with a nice tail pattern and primary window – no moulted wing coverts though for some reason.

2CY

The only 2CY I saw stood out as pretty dark on first impression, inviting confusion with LBBG. However the primary window, adult-like grey feathers and clean white rump point to Yellow-legged.

3CY

The solid adult-like grey tones makes this age class stand out, as does the extensive black on the outer primaries and primary coverts. They also have a nice clean white head and tail compared to the Herrings and LBBGs of the same age.

4CY+

Adults are pretty clear usually with their distinctive grey tone, clean white heads, rather long wings and solid black wedges in the outer primaries. The white mirrors on P9/P10 are usually more discrete than Herring Gulls and there’s a solid black band on P5 and sometimes a black mark on P4.

Other Highlights

Among the many gulls were a few identifiable argentatus Herring Gulls. The immatures show a delayed moult progression, with some 2CY looking almost like 1CY birds and lacking the grey saddle that argenteus Herring are showing by this time of year.

A few adults could be picked out on their subtly darker grey tone and primary pattern (quite Casp like) with less black and lots of white.

It was also nice seeing so many Lesser Black-backed Gulls, which are relatively hard to come by here. There were plenty of dark 4CY+ birds among them that I’d be tempted to label intermedius, if there were any way to assign this age to subspecies with any certainty.

In the 2CY below there is a notch in the primaries, presumably because of active or suspended moult. This would also be supportive of intermedius which continues to moult primaries into mid-winter. The grey saddle seems a bit pale for intermedius though.

The presence of Scandinavian taxa like argentatus Herring and intermedius LBBG implies that birds moving through here are originating from the NE.

Species Composition

Based on the numbers counted I can generate some crude estimates of species composition. These are an average of the averages from each session as, with local birds included in each session total, birds may not be unique to a single observation session.

• Herring Gull: 82.78%
• Great Black-backed Gull: 12.18%
• Lesser Black-backed Gull: 4.38%
• Yellow-legged Gull: 0.37%
• Caspian Gull: 0.29%

I suppose the conclusion from these numbers is you have to work through a lot of gulls to pull out the rarer species. Numbers moving through were very high though (around 3-5 per minute) and this resulted in about one Casp per hour of watching. If this is possible elsewhere on the south coast it could be a pretty reliable way to see this species in locations where it might otherwise be an unlikely find. Definitely going to be doing more of this gull vismig stuff later in the year and will be interesting to see how it plays out in spring.