Page 18 - Out Birding Issue 111 Summer 2023

White-billed Diver: a new regular wintering bird in the UK
It seems a long 􏰀me ago since we added White-billed Diver to the GBC List during our memorable trip to Lewis and Harris back in 2006. At that 􏰀me, this species was an official UK rarity, there being very few annual records, concentrated in the winter months in Shetland and increasingly around the Outer Hebrides in NW Scotland. By 2009, the Bri􏰀sh Birds Rari􏰀es Commi􏰁ee stopped assessing records of this species because it was becoming increasingly common around our shores, with mul􏰀ple sigh􏰀ngs each spring around the port of Ness in the Isle of Lewis.
Soon a􏰂er this, teams of ornithologists began surveys from boats as part of environ- mental impact assessment for a proposed offshore wind farm around the northern parts of the Dogger Bank in the central North Sea to the north-east of Yorkshire. The surveyors were more than surprised to keep seeing White-billed Divers on these surveys during the winter months with up to fi􏰂een being logged in some months. Because the surveys only sampled a por􏰀on of the sea, it became clear that there was a popula􏰀on of White-billed Divers in the low hundreds wintering on the Dogger Bank. The numbers were confirmed by HiDef (my former employer) digital aerial surveys of the same area. These surveys were more robust on account of the be􏰁er sampling and the photographic evidence trail. These surveys suggested that about 580 were present in January 2013 with confidence intervals of between 300 and 900.
These large numbers were previously unknown; old surveys of the area had not detected any concentra􏰀ons of divers around the Dogger Bank of any species, let alone the enigma􏰀c White-billed Diver. There are many theories about why they might suddenly have started to spend their winters in the North Sea. Nobody knows the real reason, but my favourite theory is that it is linked to mel􏰀ng polar ice caps leaving sea passages around the north of Canada and Russia clear of ice. It is inter- es􏰀ng that this sharp increase in the numbers of this species wintering offshore was not reflected in the number of records on the adjacent coastlines of the UK and the Netherlands, especially given the number of ac􏰀ve birders there.
The aerial survey data showed that the numbers of White-billed Divers would drop off sharply on the Dogger Bank during March and none were present later in spring, which 􏰀es in with birds appearing near the Port of Ness in the Hebrides. But another spring concentra􏰀on of this species emerged much closer to home for me. A local birder, who did not o􏰂en report sigh􏰀ngs, casually men􏰀oned that he had seen five White-billed Divers on the sea off the village of Portsoy on the coast of the Moray Firth in spring 2010. Local birders were unsure what to make of this, but wisely checked out the site and confirmed that they could see not just five, but six birds offshore and a further two to the west of the village. This was the most that had
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