I soon clapped eyes on a tern flying around the reservoir. Commons are regular throughout the summer but are not a regular fixture and so, even if a Common, it was a mildly notable bird.
Closer inspection of the bird showed that this was in fact a Portlandica type 1st-summer Common/Arctic, a very much more unusual sighting. 1st-summer birds, though increasingly encountered in the UK are still pretty rare, with the majority of 1st-summer Arctic & Commons spending their time in the southern hemisphere. By chance, myself, Gareth & Dan had in fact seen a 1st-summer, what we believed was Common, just a week previously on a day trip to Minsmere. Based on structure and plumage tones, we went with Common, however now in hindsight, I wish I had studied it far closer.
Getting a good grasp of all the Grimsbury bird's features wasn't easy due to the inclement conditions, as well as the fact that it spent much of it's time actively feeding over the centre of the water. Still, things that were clear were relatively uniform grey across the wings, with a thin, dark trailing edge to the primaries, clean underwing and slightly darker edged carpal area. The bill appeared all dark, while the bird had a small, white forehead, with white speckled feathering into the bird's dark cap and a largely white breast/belly with a few greyer feathers coming through. I have to say my knowledge of 1st-summer terns is pretty limited, however the relative uniform colour of the wings was bugging me a bit. Would Common show that? Due to the poor weather, I didn't bother bringing my SLR with me, however did manage a distant shot of the bird below with my bridge camera.
I had a chat with fellow local birder Steve Holiday who was also present while I was there. He was also a little unsure on the birds ID, without some more detailed literature to refer to. We tentatively went with Common for now.
The next day (07/06), I again paid the reservoir a visit and was pleasantly surprised to see that the tern was still present, again actively feeding over the water. This time I had my SLR with me and was able to obtain some far better photos. I spent quite a bit of time watching the bird, trying to take in all the features. I must've watched the bird for a good 45 minutes and not once did it rest, doing an almost figure of 8 route of the reservoir over and over again.
Back home I had a good look online for more literature on 1st-summer Common/Arctics but found it really hard to find anything clear and definitive to assist in the ID of the bird. The Collins for example, only gives only a very brief nod to 1st-summers and not really enough for conclusive ID
Looking at the photos, to me, the overall structure didn't particularly portray that pot-bellied appearance that you would associate with Arctic. The bill in the photos also appeared relatively long
I put images of the bird on Twitter as a Common, however based on feedback, the bird in fact appears to be a 1st-summer Arctic Tern! This is based on the following:
- The amount of black around the eye is beyond what a Common Tern would show
- Most importantly, all the primaries are of the same age. A Common Tern would have two generations of primaries at this time of year, having moulted the inners and retained the worn outers
So a really educational bird and a good ID challenge! Also, a cracking little record for little old Grimbo by the way!
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