Pacific Divers breed in the tundra of Alaska and northern Canada, with Baffin Island being as close as they breed to UK waters. They migrate South, wintering on both the Asian and American sides of the North Pacific ocean, as well as some on large lakes. Split from Black-throated Diver in 1985, identification can be a challenge without good views. This article ponders the status of the latest Cornish record, after first examining previous individuals.

Pacific Diver (centre) with 2 Black-throated Diver (c) John St Ledger

Cornish Records up to 2024

In Cornwall there have been two accepted records, with both birds returning for several winters. The first bird, and Britain’s third, was found on the 17th February 2007 at Penzance. It returned every winter until 2020/21, with the exception of 2011/12 and 2013/14 when it either did not return or was not seen when it did.

This bird was always seen in Mount’s Bay, between Mousehole and Perranuthnoe, with only one very notable exception when it appeared on Carnsew Pool from 19th Nov – 9th Dec 2009. This allowed for the best views of the species ever in Cornwall. Another remarkable story involving this bird occurred in February 2018, as a White-billed Diver appeared for a few days at Mousehole. Not only did this offer a remarkable chance to see Pacific and White-billed Diver in the same telescope view, potentially for the first time in Britain, it also meant that Mount’s Bay hosted all 5 of the world’s diver species in one day.

The arrival date each winter (or at least the first day it was seen and confirmed) was most commonly in November, with the earliest on the 2nd in 2009 and latest on 18th Dec 2016. The last date it was seen varied much more widely, sometimes being seen as late as May, but sometimes disappearing before Christmas. All these dates are caveated with significant uncertainty: unlike a bird present on a reservoir it is impossible to know exact arrival and departure dates as weather and distance often render it impossible to confirm the birds presence or lack of it.

The second bird was found in November 2013 at Pendower. It is loosely referred to as the ‘Gerrans Bay’ bird, but has a much broader range than the Mount’s Bay individual, with records that are assumed to be the same bird coming from Dodman Point, Maenporth and Mevagissey. This individual was also much less reliable than its Mounts Bay counterpart, although this could be a function of both observer effort and its tendency to wander further.

After being found in Nov 2013, it was seen again in Feb and Dec 2015, Nov 2017 through to March 2018, Jan 2019 and Feb 2020. The ‘arrival’ and ‘departure’ dates here have little value given the sporadic nature of the sightings. As far as I can see, the two birds were not conclusively nailed down on the same date – but given similar dates in geographically distant areas, as well as each individual’s tendency to return to the respective favoured bay, it is a widely held assumption there are indeed two different birds.

There have been no records of Pacific Diver in Cornwall accepted by BBRC after 2020. The Mount’s Bay bird was reported again in April 2021, but this was not formally accepted. There have been one or two other brief shouts, including some intriguing candidates from seawatches, but the species has not been easily available to birders in Cornwall for 4 years by 2025.

British Records

Looking briefly at British records, by 2023 there have been 11 accepted records, putting it in a rarity category that would usually be classed as ‘mega’. However, given the returning individuals mentioned above, and long stays of others, this species is not as desirable to twitchers as other species with a similar quantity of records.

The British first occurred in 2007 in Yorkshire, but remarkably a further two (including the Mount’s Bay bird) had been found within 1 month and 5 days of the first. A Pembrokeshire bird reappeared for two winters 2007-09, before one was found in Glos/Avon in Nov 2009. Shetland have recorded two in May (2013 and 2023), and while understandably accepted as new birds, it is an interesting thought if either related to winter individuals seen further South. Other records come from Northumberland and Devon in 2017, Gower in 2021 and Fife in 2023.

Boat Cove Jan 2025

The latest sighting of Pacific Diver in Britain concerns a bird found on 18th January 2025 offshore both Boat Cove and Trenow Cove, between Marazion and Perrnauthnoe. It has been aged as an adult, owing to a complete and obvious chinstrap, a pale nape and a lack of juvenile-type scapulars visible on the back. In the context of the records laid out above, it is then interesting to ponder whether this record relates to the Mounts Bay bird returning once again after 3 winters of absence.

Pacific Diver, Boat Cove, 18th January 2025 (c) Raoul Humphries

Firstly, to consider the ‘yes’ corner. The Jan-25 bird is an adult and there is nothing immediately obvious to distinguish it from any other Pacific Diver in terms of plumage or bare parts. According to AnAge, a Black-throated Diver has been recorded living to the age of 28. If the Mounts Bay bird was still alive in 2025, it would be at least 20 years old, so within the realms of possibility.

Additionally, there has been a tendency for divers in Mounts Bay to favour the Eastern end in more recent years, which suffers from much lower observer effort. Had the original bird chosen to winter offshore somewhere between Perranuthnoe and Porthleven, the chances it would have been picked up are lower than in Mounts Bay. Furthermore, Pacific Diver is perhaps the most pelagic diver species, and it could have consistently wintered much further out than before in response to changing distribution of its food source.

Switching into the ‘no’ corner, and the 3 winters without sightings is immediately significant. Unlike the more erratic Gerrans Bay bird, the Mounts Bay bird was only not seen in two winters out of at least 14, and it was seen on several occasions within most of these periods. A four-year absence would be unprecedented. Similarly, in every winter the Mounts Bay bird returned it was seen well before Christmas and thus a January first-return date would also be without precedent. The distant-wintering hypothesis is mitigated by the bird twice being seen quite close to shore on 18th January.

While Pacific Diver is still an incredibly rare bird in Britain and Ireland, records since 2007 have been consistent, with a possible increase again since 2017. Cornwall is a great county for divers in winter and is also well placed to mop up vagrants, so a new bird is well within reason. A BTO estimate of a 12 year average lifespan for a Black-throated Diver would also imply the Mounts Bay bird would be at least 50% above mean age by 2025. A final point is that 5 Black-throated Diver arrived in Mount’s Bay at nearby Trenow Cove on the 17th Jan. The suggestion here is that the Pacific may arrived concurrently, although of course from how far is pure guesswork.

To most I’ve spoken to, the evidence in the latter case appears stronger, but whether this can be suggested beyond a reasonable level of doubt remains an interesting question.

Reuben Veal, 20th Jan 2025