Pacific Diver - 2007 - first for Wales

At about 1600hrs on Friday 2 February 2007 I spotted a diver on the far shore of Llys-y-fran Reservoir whilst 'doing' the gull roost with Paul Grennard.  Paul soon got on the bird, and both began watching it through scopes.  The bird roughly displayed head and bill shape and plumage of black-throated diver, and great northern and red-throated divers were soon eliminated.  The bird did not, however, display any white rear flank patch.  It had distinctively frosty pale nape and crown (with dark border to neck sides and darker lores), and appeared quite short-bodied. 

Pacific Diver had been the subject of conversation on the journey to Llys-y-fran, as the recent national highlight had been the discovery of that species in Yorkshire.  Paul knew that lack of white rear flank patch was one of the features of that species, and we suddenly became very interested in the diver, forgetting about the gull roost almost altogether (only briefly distracted by two 1st winter glaucous gulls!).  Paul also knew that a dark chin strap was also a feature of Pacific diver.  Initially the underpats and chin appeared very clean and white, but as the diver began to settle to roost, a dark shadow could be seen on the bend of the chin when face-on in relaxed pose in the failing light.  At the time we realised that this may just be shadow, but in the mounting excitment there were some thoughts that the bird could indeed be showing a chin strap.  By now we had been joined by Richard Joseph and Malcolm Barredell.

We left the site as the light failed further, with the diver settling to roost with the gulls.

On his return to his flat Paul produced both 'Sibley' and National Geographic'.  Sibley illustrated Pacific and black-throated divers side-by-side and from these illustrations our bird most closely matched Pacific due to the lack of rear white flank patch, frosty pale nape and crown contrasting with darker border on neck sides and lores, and proportionally short body compared with black-throated.  Interesting, Silbey did not mention chin strap as a diagnosic feature in juveniles at all.

Encouraged by this information, we planned to return the next day to scrutinise the bird further, enlisting the help of Dave Astins.  Paul phoned Alan Davies of Birdline Wales to inform him that we had a brid showing characteristics of Pacific diver, and to be ready to put the news out if it was indeed confirmed the next morning.  Paul also informed the local grapevine.

The next morning the diver was located further north up the reservoir, in front of the hide.  Closer views (maybe down to 50m) in bright sunshine were obtained.  It was established that the bird in fact did not have a dark chin strap, and was clean white around the face and throat.  Some white from the belly was now riding up the lower flanks (the previous evening the flanks appeared all dark), though the bird still lacked the obvious white rear flank patch of black-throated.  A number of local birders visited the site during the morning, and some photographs were obtained (by Richard Crossen and Lydon Lomax using SLR and some 'digiscoped' record shots by me).  The debate was on!  Of particular interest was whether the bird possessed a dark vent strap or not, but it was not going to give up this detail easily (the area being generally obscured by the legs when rolling to preen).  I had to leave to give a talk in Cardiff, but DA and PG returned in the afternoon when they got brief views of a dark vent strap on three occasions during an extended bout of preening.

Meanwhile, Richard Crossen's photographs had been forwarded to Richard Millington, Martin Garner and Killian Mallarney by Alan Davies.  The general consensus was that the bird looked good for Pacific Diver, the only potentially conflicting feature being the bill looking a bit heavy.  It was thought that perhaps the photographs exaggerated the robustness of the bill, and that it was porbably with the range exhibited by Pacific diver anyhow.

More birders visited the site on Sunday 4th.  The bird was still present, but took flight mid-afternoon, as if doing a restless recce prior to departure.  During its couple of circuits around the lake the bird was scrutinised for vent detail.  This was not easy to see, but as I followed it through my scope I briefly saw a distinct dark band running right across the vent as it banked just before alighting.  The flight did give further opportunity to confirm that the rear flanks were dark.  The bird settled again to roost with the gulls.

The bird was still present on the moning on Monday 5 February, but flew off high to the south-west at 1000hrs.

Unlike the Yorshire bird, which was apparently a very distinctive individual showing all the features thought to be indicative of Pacific diver, the Llys-y-fran bird was less clear cut.  This caused great debate, particularly on BirdForum and the North American online discussion forum 'ID Frontiers', and a great amount of research on the subject (see Astins and Brown 2007).  The first stumbling block as far as many were concerned was the lack of a dark line running across the throat.  A quick re-reading of the literature soon banished this as a required feature for Pacific Diver, as of the 46 juvenile specimens examined by Reinking and Howell (1993), 21 (46%) lacked any throat strap.

Other features that set the Llys-y-fran bird apart from the Yorkshire individual were whiter cheeks (dark sullying on the face is a feature on many Pacifics), a narrower dark upper-flank band, and a longer bill, but all would appear to be within the range of Pacific diver, and where these features overlap with those of Black-throated. 

What the Pembrokeshire diver did apparently share with the Yorkshire bird was small size, pale grey-brown 'frosting' on the crown, nape and hind-neck (with dark bordering line), rounded head shape (many black-throats have flat, angular crowns), broad pale edges to the scapulars creating prominent upperpart scaling, and overall brownish tone to the upperparts.  These features are probably all subject to a certain amount of variation as will as being somewhat subjective, and it is the dark thigh that sems to be the most concrete diagnositic feture.  In Pacific diver the area of the body under the secondary coverts on the resting bird (where the wing rises towards the rear) is dark.  On the Llys-y-fran bird brown feathering in the area extended below the waterline on the resting bird, even when some white from the belly 'crept up' onto the rest of the flanks. 

In flight there was a distinct downward bulge of dark feathering directly behind the wing.  This area, more precisely termed 'anterior thigh', is white in black-throated diver, creating the distinctive white 'rear flank' patch.  We have yet to see any evidence of variation to the rule in either species; if any does come to light, then separation will prove extremely problematic!

The vent strap (prominent dark line running right across the vent), previously thought diagnostic of Pacific Diver (eg Reinking & Howell 1993, Birch & Lee 1995), would seem to be not uncommon in black-throated diver (Astins and Brown 200).

Remarkably, the diver appeared again at Llys-y-fran on 17 February 2007, remaining there up to 20 March 2007, giving more people the opportunity to see this interesting bird.

Even more remarkable was the return of the bird to the same site in January 2008, now sporting a neat dark throat-strap, silencing any remaining doubters (of which there were a few) once and for all.  Obligingly, a juvenile black-throated diver also took residence at Llys-y-fran in early 2008, giving an excellent opportunity for comparison (see Mullarney and Millington 2008).

Showing may characteristics within the overlap range of black-throated diver, the Llys-y-fran Pacific diver stirred much debate and forced a reappraisal of the identifaction criteria appearing as it did hot on the heals of the arguably more distinctive first for the Western Palearctic in Yorkshire.  It is for the reason it proved such an instructive bird.

References

Astins D & Brown J (2007). The Pacific Diver in Pembrokeshire - the second for the Western Paleractic Birding World 20

Bird A & C-T Lee (1995) Identifaction of Pacific Diver - a potential vagrant to Europe Birding World 8 (12: 458-466

Mullarney K & Millington R (2008) The Pacific and black-throated divers in Pembrokeshire Birding World 21 (2) 63-66

Reinking D L & Howell S N G (1993) An Arctic Loon in California Western Birds 24: 189-196

Sibley D (2000) The North America Bird Guide. Pica Press.

Juan Brown.

Pembrokeshire Bird Report 2007.