IDENTIFICATION ARTICLES

Least Tern (Sterna antillarum) is an uncommon to rare visitor to Hawai'i, generally in the Winter and Fall, although it has been recorded at other times too. Most individuals are recorded from the NW Chain, but a few have also been observed in the Main Islands.

Little Tern (Sterna albifrons) is a much rarer visitor to the Islands with far fewer records, all of which have so far been from the NW Chain.

The status of both species however was dramatically altered in the late1990's and 2000 when for the first time pairs of both species nested and bred in the NorthWest Hawaiian Islands - a truly amazing ornithological event!

Distribution & Movements

Least Tern (S. antillarum) breeds on the coasts of the USA and Caribbean and winters south to South Brazil and Peru. Five sub-species of Least Tern have been described, although Thompson (1992) suggests that the status of at least three of them, found in the USA, warrants reassessment. Nominate Little Tern (S. albifrons) breeds on coasts and rivers in Europe from the British Isles and Southern Baltic south to the Mediterranean and from Morocco east discontinuously to the Nile Delta, Iraq, Iran, Pakistan and India. Saunder's Little Tern (S.(a) saundersi) breeds from the Red Sea to the Seychelles. S.a. guineae breeds W and central Africa and S.a. sinensis breeds in SE Asia from Sri Lanka to New Guinea, Australia and Tasmania and Japan. Little Terns in Europe winter to South Africa and Asian birds winter as far as Tasmania. There is one breeding attempt from Saipan (Reichel et al. 1989).

Least Tern has been reported from Europe once, when a single bird was recorded amongst Little Terns at Rye Harbour Local Nature Reserve, East Sussex, England each summer between 1983 and 1992 (Yates and Taffs 1990), with the same bird being seen in West Sussex and Essex (Clifton 1992), both in England in 1991 amongst Little Tern colonies. The species has still not been officially accepted onto the British List by the British Ornithologists Union (BOU) due to the similarity of Least Tern and west African forms of Little Tern. Migrant Little Terns in SE Asia and SW Pacific have been recorded from Western Samoa, Kiribati and several islands in Micronesia (Pratt 1987).

Birds are highly migratory and their movements are not properly understood, especially as migrating individuals often consort with local birds from other populations. A bird banded in Java in 1949 was recovered in Ghana, West Africa in 1952.

Occurrence in Hawai'i

The only records of Little Tern in the AOU area involved two specimens of the race sinensis and several sight records from the NW Hawaiian Islands. Individuals have been observed in Hawai'i since at least 1976, with as many as six being recorded at once and up to twelve individuals recorded in single year. Records have fallen within the period from early March to early December with most falling between June and October.

Possible breeding took place on O'ahu in 1984, French Frigate Shoals in 1980 and Pearl and Hermes Reef in 1988 (Conant 1991). The specific identity of all the Terns, including those possibly breeding, except for the two specimens has been open to debate and not confirmed. It has been speculated by Pyle (1987), Clapp (1989) and Conant (1991) that those individuals in the Main Islands have been Least Terns and those in the Northwest Islands have been Little, but there is no evidence to support this theory, and must be solely based on distribution of each species.

During the spring and summer of 1999 Pyle, Hoffman, Casler and Mckee observed five small terns on Sand Island, Midway Atoll. Two were identified as Little Terns (S.a.sinensis) and three as Least Terns (subspecies unknown). Subsequently a pair of each nested within five meters of each other near the water catchment basin.

The first small tern on Midway was observed by Pyle and Mckee on 15th May and identified as a Little Tern, this bird was then observed more-or-less daily until 14th June. On 15th June it was joined by a second tern - this time a Least Tern. On the 18th a pair of each species was observed and were then joined by a third Least Tern on June 20th. All individuals were observed to be in full breeding plumage.

From June 20th until July 26th Hoffman and Cassler observed both species breeding and on June 28th the Little Tern pair were found to be incubating eggs near the water catchment basin, although this had almost dried up by this time. On June 30th a storm caused flooding of the basin and the nest was destroyed. However by July 14th the Little Terns had re-nested and the pair of Least Terns also nested, approximately five meters from the Little Terns, from July 11th until July 18th, when the nesting attempt failed.

The Little Terns were observed to be incubating two eggs on July 19th but this nest also failed by July 26th. The terns were observed in or near the water catchment basin until September 6th 1999.

In 2000 up to six small terns were observed on Midway during May and June, although specific identification was not made. In July Hoffman, Casler and McKee observed up to three adult and one sub-adult Little Terns. One pair of adults nested successfully and by July 21st had successfully fledged at least two chicks, and these were observed until September 20th.

Although this is the first confirmed successful breeding of sterna Terns in Hawai'i it seems likely from previous observations and observations of juvenile birds in the State that breeding has occurred before.

Identification of the Midway Terns

All identification features and vocalizations that follow of the Midway birds are by Pyle, Hoffman, Casler and McKee as published in North American Birds 55:1, pp.3-6.

Voice: Little Tern - single-note nasal "chent" call notes. Also dry chatter reminiscent of Ruddy Turnstone when courting. On one occasion a soft warbler-like "chwip".

Least Tern - double-noted "ch-dic" and when courting an up-slurred, guttural "rre-it".

Apparently the vocalizations were very distinct, allowing identification of individuals and species.

Size and tail Length: The two Little Terns were 10-15% larger than the three Least Terns and had longer outer retrices. Within each species there was some size variation (probably sex-specific), but the smaller of the 2 Little Terns was still approximately 10% larger than the largest Least Tern.

Plumage: The Little Terns had white rumps and tails, contrasting crisply with the pale gray lower backs. The Least Terns had gray rumps and tails, except for the outer two-three pairs of retrices, which did not contrast with the back color. In addition the two Little Terns showed paler upper and under surfaces to the primaries than the three Least Terns. There were no differences in head plumage or bill and leg color.

"The difference in the color of the primaries on the Midway birds resulted from the more extensive black shading to the inner webs of the middle primaries (p4-p7) on the Least Terns than on the Little Terns. This potential fieldmark has not been mentioned for the separation of these two species, although variation in the color of the outer primaries has been used to assign sub-specific status among population of Least Terns (see Patten and Erickson, 1996), with eastern sub-species (S.a. antillarum) averaging paler outer primaries than western and southern subspecies (browni and mexicana). The amount of dark on the primaries is small sterna is affected by the combination of a complex molt among these feathers and wear (Cramp 1985, Chandler and Wilds 1994, Higgins and Davies 1996). It is possible that the differences noted on the Midway birds were related to differences in the previous molt of primaries.

There is also variation in the colour of the shafts of the outer two primaries: typically white to horn in Asian Little Tern (S.a.sinensis) and black, concolorous with the webs of these feathers, in Least Terns and nominate Little Terns from Europe (Cramp 1985, Clapp 1989, Conant et al. 1991, Higgins and Davies 1996)."

"Note also that the tails of sinensis are longer than in other subspecies of Little Tern (Cramp 1985, Higgins and Davies 1996): thus, tail length may not be as useful in the separation of the Least Tern from European and African forms of the Little Tern."

Pyle et al. (2001) state that the contrast between the rump color and back colour is likely the best plumage feature to separate Little Terns from Least Terns in breeding plumage. However, individuals of some populations do exhibit gray rumps too. Pyle et al. cite those individuals from the Phillipines as having slightly grayer rumps and tails than other forms or sub-species, although go on to say that the back color is usually contrasting with the rump and tail color.

Saunder's Tern (S. saundersi) differs from S. albifrons by its smaller size and paler mantle and wings, contrasting more strongly with the darker outer primaries, of which there are normally three, not two, the result of arrested moult at p7 rather than p8. The darker gray of the rump, concolorous with the mantle, extends to the central tail feathers, which could result in confusion between Saunder's and Least Terns, however, Saunder's also shows olive or reddish-brown legs, with any yellow present restricted to the feet or rear tarsus (Chandler and Wilds 1994). In Saunder's Tern the white on the forehead is usually square-shaped, rather than indenting into the black forehead in Little and Least Terns, photographs showing this feature well can be seen in the article by Chandler and Wilds (1994) in British Birds.

According to Chandler and Wilds (1994) many of the features supposedly distinguishing Saunder's from S. albifrons intergrade with characters of that species. Areas for possible confusion do exist! It is also worth noting that Saunder's Tern is regarded as exclusively marine.

It seems true to state though that perhaps some other races of Little Tern show darker gray rumps and tails than the ones discussed with here, for example the west African race of Little (guineae) appears to show a gray rump and a voice similar to Least Tern - one reason why the BOU has not yet accepted Least Tern onto the official British list.

There seems little doubt however, that the birds recorded at Midway are of both species and as pointed out by Pyle et al. increases the known breeding ranges of both species by over 4000km - a truly outstanding observation!

The 2001 Fall Migration issue (54:1) of North American Birds has several good comparison photographs of both the Little and Least Terns that were present on Midway, as well as the complete paper by Pyle et al. on their occurrence.

Below left and right: Little Tern exhibiting greyish rump, Netherlands 2002. Photographs © by Michiel Noback.

The above individual was found moribund in the Netherlands and initially tentitively identified as a Least Tern by the finder, Michiel Noback, due to the greyish rump, it was weak when he first saw it and within the hour had died. It appears from the dead specimen that it has a grey rump and upper tail consistent with Least Tern (although the flash has overexposed part of the photo) and it seemed reasonable to presume it was one, however as the photo was taken in Holland and so would be only the second for the Western Palearctic (assuming that a bird in England from 1980-1990 was this species, although still not accepted by BOURC yet). Some races of Little Tern can show grey/greyish rumps but apart from the African race these are in Asia (and so just as unlikely in Europe). I don't think anyone has sorted out some African birds yet, but with just two grey-rumped Least/Little Terns recorded in Europe so far they presumably do not wander very far or indeed do not actually have grey rumps!

Details and photos of the bird were passed to Peter Pyle in the United States for comment. Peter kindly commented as follows: " The back/rump/uppertail covert contrast is difficult to assess based on the jpeg and this monitor - it appears somewhat intermediate between LETE and LITE to me. In the field, the LITE's on Midway appeared to have a straight cutoff between the gray back and white rump/uppertail coverts and tail, wherase the LETEs had a gray back and rump that also spilled over into some or all of the uppertail coverts and perhaps the central rectrices. The photos of the LETE from England matched the LETEs on Midway well. Although the Dutch bird appears to have some gray in the rump, the rest of it looks more like the Midway LITEs to me. I don't recall differences between nominate LITE and sinensis, the subspecies we were presumably dealing with on Midway, and this might have something to do with it. I'd be interested in the assessment based on the specimen."

Michiel obtained measurements of the bird:
wing-span: ~480 mm
length: 218 mm
black tip (on upper bill only): ~1 mm
white on forehead, from base of bill to above eye: 14 mm
height of black lore: 5 mm just in front of eye; 3,5 mm at base of bill
outer two primaries black
center of tail (light) grey, outer two feathers whitish
underparts (almost) white; side of breast very light grey
weight (although not relevant considering the birds'condition): 45 gram

The pictures are on Michiel's website: http://www.noback.net/michiel (Search under dwergstern)



References

CHANDLER, Dr. R.J. and WILDS, C. 1994. Little, Least and Saunder's Terns. British Birds Volume 87. No.2. pp.60-66.

CLAPP, R.B. 1989. First record of the Little Tern , Sterna albifrons, from Hawai'i. 'Elepaio 49:41-46.

CLIFTON, J. 1992. Least Tern at Colne Point EWT Reserve - 29th June to 1st July 1991. Essex Bird Report (1991): 120:121.

CONANT, S.; CLAPP, R.; HIRUKI, L. and CHOY, B. 1991. A new Tern (Sterna) breeding record for Hawai'i. Pacific Science Volume 45. No. 4: 348-354.

CRAMP, S. ed. 1985. The Birds of the Western Palearctic, VolIV. Oxford University Press, Oxford.

HARRISON, P. 1983. Seabirds an Identification Guide. Helm, London.

HIGGINS, P.J. and DAVIES, S.J.J.F., Eds. 1996. Handbook of Australian, New Zealand and Antarctic Birds. Volume 3. OUP, Oxford.

MASSEY, B.W. 1976. Vocal differences between American Least and the European Little Tern. Auk 93. pp. 760-773.

NOBACK, M. 2002. Website: http://www.noback.net/michiel .The Netherlands.

OLSEN, K.M. and LARSSON, H. 1995. Terns of Europe and North America.

PRATT, H.D., BRUNER, P.L. and BERRETT, D.G. 1987. A field guide to the birds of Hawaii and the tropical Pacific. Princeton University Press, Princeton, New Jersey.

PYLE, P. , HOFFMAN, N., CASLER, B. and McKEE, T. 2001. Little and Least Terns breeding on Midway Atoll: Identification, range expansions and assortative breeding behavior. In North American Birds 55:1. Colorado.

REICHEL, J.D., ALDEN, D.T. and GLASS, P.O. 1989. Range extension for the Little Tern in the Western Pacific. Colonial Waterbirds 12:218-219.

THOMPSON, B.G. and SLACK, R.D. 1983. Moult-breeding overlap and timing of pre-basic moult in Texas Least Terns. Journal of Field Ornithology, 54: 187-190.

YATES, B. and TAFFS, H. 1990. Least Tern in East Sussex - A new Western Palearctic bird. Birding World 3/6. pp.197-199.

Christian Melgar, West Sussex, England. 2001.

---