BARC Submission: Antarctic Tern Sterna vittata

Pedra Branca Pelagic (Tas) 29 July 2008

Submission to Birds Australia Rarities Committee

Simon Mustoe, 39 The Crescent, Belgrave Heights, VIC 3160, Australia

1. Introduction

An Antarctic tern was seen on the 29th July, 12:00-12:20 at 44.071613°S, 147.302337°E about 30 miles south of Southport, Tasmania. It spent a few minutes behind the vessel and was photographed a number of times over two short periods between 12:03-12:04 and then again when it returned from 12:15-12:18 (section 6). It was only at the end of the second encounter that I became aware of the bird at the back of the vessel. As I moved aft, I drew attention to it because the size and shape looked odd, but not soon enough. It flew off and I only had time to take a couple of distant photos. I did not review these until my return from Tasmania a few days later. The bird was initially identified as a White-fronted Tern Sterna striata.

Apart from White-fronted Tern, the main confusion species, Arctic Tern S. paradisaea and Common Tern S. hirundo, were expected to be breeding in the northern hemisphere at this time of year, although it is possible that subadult birds could remain south. This submission describes the Antarctic Tern and reviews existing information on identification, necessary to separate it from the above species.

The manner of discovery was not dissimilar to the situation off Port Fairy in July 2005...(images by Rohan Clarke). For comparison, there are also photos of breeding plumage birds from Macquarie Island (care of Rohan Clarke). If accepted by the Birds Australia Rarities Committee, this would be a first record for Tasmania (Bill Wakefield, pers comm.) and possibly only the second "mainland" record east of Kangaroo Island, where a number of birds were present in late 2006. It is possible that Antarctic Terns occur regularly off southern Tasmania, as they breed on Macquarie Island.

2. Description

2.1 Moult and Age

The bird was in non-breeding adult plumage and undergoing wing and tail moult. There were no retained immature plumage feathers. With reference to the images below, primaries P7-P10 are older, retained feathers, whilst P1-P6 have been recently replaced. P6 is still growing, being considerably shorter than P7 (Photo A). Tail moult in Arctic and Common terns begins with T1 through to T3, then T6 and finally T4 and T5. It is likely that this same moult sequence applies to Antarctic Terns (Danny Rogers, pers comm.). T4 and T5 appear to be missing altogether (Photo H), so the tail streamer comprises just one feather, T6.

2.2 Features

2.2.1 Overall Appearance

The bird's overall appearance is like one of the "commic" terns, with a medium-grey back and upperwings, white throat, black cap and white rump, tail and undertail coverts. Because I did not see the bird for more than a few seconds, I don't have any recollection of the flight style. From the photographs though, it appears relatively chunky, with a large head. The wings are not disproportionately long (e.g. as with Arctic Tern).

2.2.2 Underside Mottling

As shown in Photos C, E and G, a substantial area of the belly is mottled grey, similar in colour to the back. This mottling covers most of the underside from between the feet all the way up to the chest, including the flanks, and some of the throat feathers. On the head, this contrasts with paler cheeks. The underside mottling also extends up just infront of the leading wing join, becoming continuous with the grey saddle. Between this and the posterior edge of the black cap is a thin white collar (Photo G & H).

2.2.3 Head (Photo D)

The lores and forehead are white / mottled grey. From a distance, the anterior edge of the black cap appears to be about in line with the front of the eye. There is a large squarish black "smudge" (not clearly defined) in front of the eye and a narrow line of dirty-lack feathering below the eye. This and the lower border of the black cap, behind the eye, are not clearly demarcated from the white cheeks. By contrast, the back edge of the cap is clearly demarcated, forming a distinct border at the nape, narrowly separated from the grey back by a white line. The black cap extends forward to about the line of the eye and merges with the pale forehead in a series of prominent streaks. The white streaking is longest and extends furthest back along the head at the centre of the crown, reaching to just behind the eye.

2.2.4 Wings and Tail

Overall, the wings are medium-grey with a pale trailing edge along the secondaries, a very narrow (almost non-existent) pale trailing edge on the inner primaries and a dark trailing edge towards the wing tips (P7-10). The pale trailing edge is thickest from the third tertial to the fifth secondary, forming a pale panel in the centre of the wings (Photo G & H). However, this may be a manifestation of the moult process, rather than a real colour difference. The outer primaries have distinctly dark leading edges (particularly P1). The tips of P7-P10 are very thickly edged dark-grey and this colour hooks back to extend about one fifth of the way down their trailing edge. This gives the outer primaries the appearance of dark feathers with distinct pale centres.

The tail and rump is mostly pure white. The white extends onto the rump and is abruptly divisible from the grey back. The tail itself is deeply forked, although because T4 and T5 are missing (see Moult and Age, above), the centre of the fork is square (Photo H). The one tail feather that forms the streamer (T6) is long and appears to have a pale grey outer web.

2.2.5 Bare Parts

The bill is almost completely dark red and quite long, with a distinct gonydeal angle. The feet are similar in colour.

3. Comparison of Identification Features Between Antarctic Tern and Confusion Species

Table 1 summarises published identification information for Antarctic Tern and the three relevant confusion species. Cells coloured white largely match the description of the the bird given above (section 2). Cells coloured grey contain features that largely do not match the description above. Note, the review in Table 1 is close to the precise wording given in the references (section 5).

Table 1: Review of descriptions of medium sized confusion species

	Antarctic Tern	Arctic Tern	Common Tern	White-fronted Tern
Lores, forehead and crown	Lores and forehead white, merging into dark-streaked forecrown (3). Black band extending from in front of eye and broadening across ear coverts to nape, and with black streaking on hind crown, or black streaking or mottling over most of crown (5).	Distinctive. White lores, forehead and forecrown (3). Black streaking on rear-crown and black band extending from in front of eye, through ear coverts to nape (5). Black cap recedes to behind eye (2).	Forehead and forecrown white, to level with or slightly behind eye. Large black patch in front of eye (5).	Forehead and forecrown white, extending to mid or rear-crown, to about level of eye (5), where spotted black (3). Black cap reaches forward to just above eye (2,3). Prominent squarish black patch in front of eye (5).
Bill	Bright to dark red or red-black with small red tip and small black area on base of mandible in some (5). More prominent gonys than Arctic Tern.	Black (5).	Black, rarely with red tinge at base or up to basal half, very dark red (5).	Black (5) and long (2) (almost disproportionately so).
Legs	Bright red to dark red or brown-red (5).	Dark red-brown to dull black (5). Extremely short.	Black with faint red tinge to orange-red (5).	Short, black or reddish-black (2).
Body	Grey mottled / smudged underparts (3,5). Medium-grey upperparts.	White / slightly grey underparts in non-breeding plumage (2). Medium-grey upperparts.	White below in non-breeding plumage (2). Medium-grey upperparts.	White below (2), very pale pearl-grey above (3).
Primaries	Small amount of black on tips of outer primaries forming thin black line along leading edge of outer primary (5). Inner portions of outer wing appear translucent, except for diffues grey trailing edge on 5-6 (3,5).	Small amount of black on tips of outer 3-4 primaries. All of outer wing appears translucent against the light, except for thin well-defined dark trailing edge (2,3).	Outermost 3-4 dark grey primaries, creating dark wedge (3) contrast with paler grey inner primaries, which appear as a translucent patch against the light (2,3). Diffuse dusky trailing edge to tip of underwings (2,3).	Mainly white / pale grey with distinctly duskier outer primaries (3).
Rump and tail	White with grey sides (5).	Thin dark outer-edge to tail (2,3). Rump to tail mainly greyish-white (3).	In non-breeding, rump light grey, grading to paler grey-white on uppertail coverts. Contrast only slightly with upperparts (5). Dark grey sides to tail (2,5).	Pale grey rump and tail, almost no contrast with saddle and upper wings (5).
Jizz	References describe Arctic Tern but not Antarctic Tern. By contrast, Antarctic Tern should be proportionately shorter-winged (less narrow and tapered), chunkier bird with a proportionately larger head, larger bill.	Slim, long-tailed and long-winged (2). Wings appear proportionately short and narrow; more neckless appearance, small head and bill (3). Wings appear to be on front of centre of body (4).	More gull-like than Arctic. Larger head, shorter wings (3).	Relatively long pointed wings. Slim build (3).

4. Discussion

Based on identification characters from the literature (Table 1), the bird matches all the features for Antarctic Tern. It is most similar in build to Common Tern but the grey mottling on the breast and the pale centres to the outer primaries rule out this species. White-fronted Tern is ruled out on almost all characters: the base colour of the bird is far too dark, the cap extends too far forward on the crown, the bird is too compact and has a red, rather than black bill.

Comparison with Arctic Tern is more difficult. Bare part colour and overall plumage characteristics are similar. Many of the differences are relative, rather than absolute. Although in non-breeding birds, one would not expect Arctic Terns to have mottled grey on the breast, grey underparts is a character of breeding birds and according to Heather and Robertson (1996), can be found in non-breeding birds. Nevertheless, this feature is probably too prominent for Arctic Tern.

There are several compelling separation features. 1) the overall shape of the bird is too large-headed and bulky for Arctic Tern; 2) the grey in the outer primary tips is too extensive. The thickness of the dark edge, particularly at the trailing edge, is much greater than in Arctic Tern. The primaries on Arctic Tern are more 'translucent' because the dark edges are only narrow so the outer primaries appear mostly white. The translucent centres to the primaries in the Antarctic Tern are much reduced and less obvious in the open wing, so the outer primaries appear mostly dark; 3) the black cap extends too far forward. In Arctic Tern, the white on the forehead of non-breeding birds would extend well back onto the crown (rather like White-fronted Tern); and 4) the bill is also longer and slightly heavier than Arctic Tern, with a prominent gonydeal angle.

5. References and Bibliography

1. Enticott, J. & Tipling, D. (1997) Photographic Handbook of the Seabirds of the World New Holland, London.
2. Heather, B. & Robertson, H. (1996) The Field Guide to the Birds of New Zealand Viking, Auckland.
3. Shirihai, H. & Jarrett, B. (2002) A Complete Guide to Antarctic Wildlife Alula Press, Degerby.
4. Mullarney, K., Svensson, L., Zetterstrom, D. & Grant, P. (1999) Collins Bird Guide Harper Collins, London.
5. Higgins, P. J. & Davies, S.J.J.F. (1996) Handbook of Australian, New Zealand and Antarctic Birds Volume 3 Snipe to Pigeons Oxford University Press, Melbourne.

6. Images

Note, it is understood that images were received as close to raw appearance as possible, though they have been cropped and zoomed. A slight adjustment was made to the colour histogram using Adobe Lightroom, mostly to reduce clipping of the blacks. The day was heavily overcast, which is why the images appear dark. They have not been artifically lightened, in an effort to remain true to the original images.