The Aleutian Tern Sterna aleutica:
a new seabird species for South Korea and Korean waters
Nial Moores, Birds Korea, August 2004

An Aleutian Tern Sterna aleutica was seen well but briefly from a hydrofoil in open sea at approximately 70 Km East-Southeast of Socheong Island (at very approximately 37° 30' N, 125° 00' E), on August 23rd 2004 by Nial Moores. No photographs were taken. The species is therefore admitted into category 2 of the Birds Korea unofficial checklist (as an observation lacking supporting evidence, to be formally reviewed by a future rarities committee).

Opportunities for sea-watching in South Korea are somewhat limited, as there are few distinct headlands, and most outer islands are nowadays served by high-speed ferries. Although outside access on most such ferries are highly restricted, NM is usually given permission by the crews to sea-watch from outside. On the afternoon of August 23rd, from outside and to the rear of the high-speed hydrofoil traveling broadly southeast from Socheong Island to Incheon, sea-watching conditions were extremely good, with very light seas, and sunshine: conditions adequate, for example, to see clearly the upperwing bar on several feeding Swinhoe's Storm Petrel Oceanodroma monorhis. After approximately one hour or ca 70 km East-southeast of Socheong (and perhaps ca 30 Km southwest of the coastline of Kangnyong Gun, DPRK), a lone “medium-sized” tern, rather similar to a Common Tern Sterna hirundo in overall appearance, was observed sitting on a bright white buoy, at closest less than 50 m from the ferry. The bird and the buoy were first seen to the front and to the north (or left) of the boat, with the afternoon sunshine coming from the rear and to the right (the southwest). The tern was facing right, but turned towards the boat as the boat passed, illuminating its head pattern, and causing its fairly long-looking apparently black bill to glint in the sunshine. Although decent views were both necessarily very brief (less than 10 seconds close from the rear of the boat), and only through binoculars, the tern looked elongated (like a Common), with its rear-end or primaries rather upended, and clearly had very dark slate-grey upperparts (appearing darker than expected in longipennis Common Tern), paler underparts, and an apparently whitish breast becoming much duller or grey-washed towards the belly (though all shades on the underparts were very difficult to judge accurately due to the combination of limited time, strong sunlight and the reflected white light of the buoy). Focusing on the head pattern, it completely lacked the expected paler lower “face” of a Common Tern. Instead, the head pattern appeared much more contrasting and distinctive, immediately recalling breeding-plumaged Little Tern Sterna albifrons, with a clean black cap (perhaps very slightly ragged-edged towards the fore?), a very extensive white forehead (with white apparently coming back to above the eye), and a distinct black line from the cap, apparently through the eye and certainly across the lores, becoming less distinct (even broken perhaps?) immediately before the bill. These features on the head were double-checked as the boat passed the buoy, until distance was no longer adequate to see any details. Although not seen in flight (or photographed), these features in combination are believed diagnostic of an adult-type Aleutian Tern Sterna aleutica (starting to move from breeding into non-breeding plumage).

Somewhat frustratingly, of a further ca 921 terns logged during the next 90 minutes on the same ferry crossing (including a single flock of approximately 380), only 200 or so were seen close enough for a reasonably confident identification, and all of these were longipennis Common Tern (with some still in dark-grey breeding plumage, many in intermediate plumages, and only a few juveniles). One quite distant pale individual tern, however, appeared to show in flight a dark trailing edge to the underside of the secondaries combined with rather extensive dark on the underside of the primaries. Both features are indicative of non-breeding plumage Aleutian Tern.

Breeding in eastern Siberia (e.g. in coastal Bering, Okhotsk and South Chukchi Seas) and wintering southward, the Aleutian Tern is likely to occur rather more regularly in Korean waters than this single first record suggests. The species for example is regularly observed in Hong Kong (perhaps most regularly in August and September [Paul Leader, pers. comm.]), where there is a concentration of experienced observers and also ferries to carry observers into preferred areas. By contrast there is very limited coverage of sea areas by either Korean ornithologists or visiting birders, and thus still rather limited knowledge of migrant seabirds in Korean waters. Recent observations from ferries by NM and others, however, especially between Incheon and Socheong and to a lesser extent between Gunsan and Eocheong, Mokpo and Gageo Island, indicate significant movements of a number of seabird species. Several of these species are presently suspected of undertaking an overland crossing on a route across the southern DPRK into the northern part of the South Korean Yellow Sea. This presumption is somewhat supported at least by satellite transmitters placed on Yellow-billed Loons Gavia adamsii in Alaska, that then moved from the East Sea to waters near Socheong and off the Shandong Peninsula.

In the case of the Aleutian Tern, it seems plausible that it too might have undertaken an overland crossing. There was a more than eight-fold increase in the number of terns recorded on August 23rd when compared with the 111 observed on the same crossing on August 18th. This increase seems likely to be linked to the passage of typhoon Maegi through the Korean Straits into the East Sea (“Sea of Japan”) on August 19th and 20th, a system that produced South Korea's first record of Great Frigatebird Fregata minor. Following the typhoon, heavy rains became established over much of South Korea. To the north of this rain system a drier, clearer airflow became established, with the southern edge of this northeasterly air stream lying directly through the area. This weather system stimulated significant movements of passerines and even shorebirds on the night of August 22nd and early 23rd (see Latest Bird News).

NOTE: Lacking either previous experience of the species or adequate literature, NM contacted Paul Leader in Hong Kong on August 25th about the occurrence and appearance of Aleutian Tern there. In autumn in Hong Kong, Aleutian Tern numbers regularly peak in August, with a second peak also possible in the second or third week of September. Most individuals are adults, and many are either already in non-breeding plumage or are moulting from breeding to non-breeding plumage. Apparently, many individuals in mid to late August in Hong Kong therefore show features compatible with the individual described above (e.g. showing a perhaps broken loral line, and paler underparts below than breeding plumaged individuals). Additionally, many are also found loafing on floating garbage (leading to their alternative and derogatory nickname of “Polleutian Tern”!), at which times many sit with their primaries angled rather strongly upwards. This distinctive posture was noted in the bird of August 23rd, though it was considered at the time (and presently) too difficult to determine whether this was a reaction to the passing boat, or due to some structural differences between Aleutian and Common Terns.

Update: 02 September 2004:

Will Duckworth has provided the following references, and makes the comment that it seems worth considering how: "Aleutian Terns are prone to be overlooked even in well watched areas and then when people find out where and how to see them, suddenly they become quite common."

  • Kennerley, P. R., Leader, P., and Leven, M. R.1993. Aleutian Tern: the first records for Hong Kong. Hong Kong Bird Report 1992: 107-113.
  • Kennerley, P. R., and Ollington, R. 1998. Aleutian Terns in South-East Asia. Oriental Bird Club Bulletin 27: 34-41
  • Ollington, R. F. and Loh E. 1996. Aleutian Terns wintering area found. Birdline Singapore Monthly Newsletter 42: 5-10."