Part 61

Sterna candida Gmelin, Syst. Nat., 1, pt. 2, 1789, p. 607. (Type locality, Christmas Island.)

Gygis candida Finsch, Ibis, 1880, p. 220 (Taluit); Saunders (part), Cat. Birds British Mus., 25, 1896, p. 149 (Marshalls); Schnee, Zool. Jahrbücher, 20, 1904, p. 390 (Marschall-Inseln).

Gygis alba Finsch, Ibis, 1880, pp. 330, 332 (Taluit); Wiglesworth (part), Abhandl. und Ber. Zool. Mus. Dresden, no. 6, 1890-1891 (1891), p. 78 (Marshalls); Oustalet (part), Nouv. Arch. Mus. Hist. Nat. Paris, (3), 8, 1896, p. 58 (Saypan, Pagan, Agrigan, Marshalls); Safford, Guam, 1912, p. 19 (Guam); Strophlet, Auk, 63, 1946, p. 537 (Guam); Baker, Condor, 49, 1947, p. 125 (Guam); Stott, Auk, 64, 1947, p. 525 (Saipan); Baker (part), Smithson. Misc. Coll., vol. 107, no. 15, 1948, p. 57 (Guam, Rota, Saipan).

Gygis alba kittlitzi Hartert, Novit. Zool., 5, 1898, p. 67 (Saipan, Guam); Seale, Occ. Papers Bernice P. Bishop Mus., 1, 1901, p. 21 (Guam); Safford, Osprey, 1902, 66 (Marianas); idem, The Plant World, 7, 1904, p. 267 (Guam); idem, Contr. U. S. Nat. Herb., 9, 1905, p. 80 (Guam); Mathews (part), Birds Australia, 2, 1912, p. 443 (Marianas); Prowazek, Die deutschen Marianen, 1913, p. 100 (Marianan); Cox, Island of Guam, 1917, p. 22 (Guam); Ridgway (part), Bull. U. S. Nat. Mus., 50, pt. 8, 1919, p. 559 (Mariannes); Kuroda, Avifauna Riu Kiu, 1925, p. 193 (?Mariannes); Yamashina, Tori, 7, 1931, p. 410 (Saipan); Yamashina, Tori, 7, 1932, p. 409 (Iringlab, Namo, Aruno); Hand-list Japanese Birds (part), rev., 1932, p. 196 (Guam, Tinian, Saipan, Pagan, Agrigan, Jaluit, Mille, Aurh, Wotze, Likieb, Mejit); Yamashina (part), Tori, 10, 1940, p. 678 (Assongsong).

Gygys alba Wheeler, Report Island of Guam, 1900, p. 13 (Guam).

Gygis albus kittlitzi Kuroda (part), in Momiyama, Birds Micronesia, 1922, p. 50 (Guam, Saipan, Pagan, Agrigan, Marshalls).

Leucanous albus kittlitzi Mathews (part), Syst. Avium Australasianarum, 1, 1927, p. 143 (Marianne).

Gygis alba microrhyncha La Touche (part), Handbook Birds Eastern China, 2, 1933, p. 335 (Marianne).

Gygis alba candida Bryan, Guam Rec., vol. 13, no. 2, 1936, p. 24 (Guam); Hand-list Japanese Birds (part), 3d ed., 1942, p. 219 (Guam, Tinian, Saipan, Pagan, Agrigan, Assongsong, Jaluit, Mille, Aurh, Wotze, Likieb, Mejit); Downs, Trans. Kansas Acad. Sci., 49, 1946, p. 94 (Tinian); Borror, Auk, 64, 1947, p. 417 (Agrihan).

Geographic range.--Northern Pacific from Bonins and Marianas east to Wake and Hawaiian Chain, south to Marshall, Phoenix, Christmas and Fanning islands (see figure 12). In Micronesia: Mariana Islands--Guam, Rota, Tinian, Saipan, Pagan, Agrihan; Marshall Islands--Jaluit, Mille, Aurh, Wotze, Likieb, Mejit, Eniwetok, Bikini, Kwajalein.

Characters.--Adult: A small tern with ivory-white plumage except for black, narrow, orbital ring; shafts of primary quills dark brown; shafts of tail feathers blackish; bill black with bluish base; tarsus dark bluish with yellowish webs; iris and skin black.

Immature: Resembles adult, but with light brown mottlings on upper parts, especially on the mantle; feathers softer, bill shorter.

Measurements.--Measurements are listed on table 23.

Weights.--The NAMRU2 party obtained weights of 11 adult males from Guam and Rota as 110 (97-124); weights of 6 adult females from Guam as 108 (100-116). These specimens were taken from May to October, 1945.

Specimens examined.--Total number, 41 (23 males, 14 females, 4 unsexed), as follows: Mariana Islands, USNM--Guam, 20 (May 24, 29, June 6, 8, 14, 15, 16, 18, 23, July 10, 19, 20)--Rota, 2 (Oct. 19, 27)--Saipan, 1 (Sept. 26); AMNH--Guam, 4 (March 7, 9, 20)--Tinian, 1 (Sept. 8)--Asuncion, 4 (Jan. 1, 18, 25); MCZ--Saipan, 3 (Jan. 7, March 20, April 17); Marshall Islands, USNM--Bikini, 6 (Feb. 27, March 2, 16, 19).

Nesting.--Gygis alba does not construct a nest but places its single egg rather precariously in the crotch of a branch in a tree (or on rock). In Micronesia nesting activities have been observed at various times of the year. Yamashina (1932a:409, 410) reported on eggs taken in the Marianas at Saipan on February 2 and in the Marshalls at Arhno on September 26, at Iringlab on October 21 and at Namo on October 19. At Guam a pair of White Terns was seen in a large tree on March 27, 1945, by the NAMRU2 observers. Because of their behavior, it was suspected that they had an egg or young in the tree. Further inspection revealed, on March 31, a downy young sitting in the tree. The young bird was attended by the parents until it began to fly on April 17. Hartert (1898:68) reports that eggs of the White Tern were taken at Saipan on July 28 and August 11. Morrison obtained a male nestling on March 16 and eggs on March 22 at Bikini in 1946.

Remarks.--The White Tern is usually restricted to the remote islands in the Pacific, Indian and South Atlantic oceans; there, according to the latest treatment, which is that of Peters (1934:348, 349), six subspecies are recognized. In studying the geographical variation of the species, the writer has examined 595 adult specimens, including previously unstudied material collected by the Whitney South Sea Expedition, which is deposited in the American Museum of Natural History.

This ivory-white species presents an unusual problem in that there are few characters available to distinguish the subspecies. Measurements of taxonomic value include those of the wing, tail, exposed culmen, and depth and the shape of the culmen. There appears to be no significant secondary sexual difference between males and females, and measurements of the two sexes are combined. The chief problem within this species seems to hinge on how to classify isolated, but relatively similar, populations. The examination of the large series of specimens from the Whitney collections has yielded more complete information to assist in the solution of this problem.

Gygis alba alba (Sparrman) of the South Atlantic Ocean (Fernando de Noronha, South Trinidad, Ascension, and St. Helena islands) and G. a. monte Mathews of the Indian Ocean (Seychelles, Aldabra, Mascarene and Chagos islands) are isolated populations. Specimens examined are those which have previously been studied by other workers; measurements are shown in table 23.

With the exception of G. a. microrhyncha, G. a. monte has the smallest average length of wing of all of the subspecies of G. alba. In G. a. alba the length of wing as well as most of the other measurements differ but slightly from those of some of the populations in the Pacific area although the slender bill of the Atlantic bird is a distinctive character, as pointed out by Murphy (1936:1166).

TABLE 23. MEASUREMENTS OF SUBSPECIES OF Gygis alba FROM THE ATLANTIC AND INDIAN OCEAN AREA

Column headings:

A: No. B: Wing C: Longest tail feather D: Shortest tail feather E: Exposed culmen F: Depth culmen G: Tarsus

===================+====+=======+======+=====+=====+=======+========= SUBSPECIES | A | B | C | D | E | F | G -------------------+----+-------+------+-----+-----+-------+--------- Gygis alba alba | 24 | 246 | 99 | 71 | 40 | 8.0 | 14.5 | |239-256|93-111|68-77|35-44|7.5-9.0|13.0-16.5 | | | | | | | Gygis alba monte | 35 | 232 | 106 | 71 | 39 | 8.5 | 13.5 | |224-244|98-116|64-81|37-44|8.0-8.5|12.5-14.0 -------------------+----+-------+------+-----+-----+-------+---------

The taxonomic position of the White Terns of the Pacific area has been one of uncertainty for a long time; as Peters (1934:349) puts it, "It is obvious that the last word on the Pacific races of Gygis has not yet been said." A principal feature of the problem in this region is the presence in the Marquesas of a well-marked subspecies, G. a. microrhyncha, virtually surrounded by a wide-ranging and relatively undifferentiated form, G. a. pacifica (Lesson) (see figure 12). The small cormorant (Phalacrocorax melanoleucus brevicauda Mayr) from Rennell Island, Solomons, is another example of a distinct form surrounded by a widely distributed subspecies.

In all, 55 adult specimens of G. a. microrhyncha have been examined from the following islands in the Marquesas Group: Mukahiva, Eiau, Motane, Hivaoa, Uapu, Tahuata, Uahuka, Fatuhiva. The measurements are listed in table 24, and show that the White Tern in the Marquesas is a much smaller bird than the other subspecies and has a shorter bill, wing, and tail. The tail possesses a shallow fork as compared with the deeper fork of the tail of other subspecies. In addition, the depth of the culmen averages two millimeters less in the subspecies in the Marquesas. The presence of a wider, black eye-ring is also a distinguishing character in this subspecies.

Gygis a. microryhncha was for a long time treated as a species distinct from G. alba but has recently been considered as a subspecies G. alba by Peters and others. On the islands of Hatutu and Motane in the Marquesas, the Whitney South Sea Expedition obtained some birds which appear to be intergrades between the two subspecies of White Terns in the area. The measurements of nine birds which show intergradation between G. a. microrhyncha and G. a. pacifica are listed in table 24. Probably the Marquesas population is tending toward complete reproductive isolation.

Peters (1934:348, 349) recognizes three other subspecies from the Pacific area: G. a. rothschildi Hartert from Laysan, Lisiansky, and Krusenstern islands; G. a. candida (Gmelin) from "the Carolines east to Christmas Island and south to the Tonga and Society Islands"; and G. a. royana Mathews from Norfolk and the Kermadec Islands. Birds from Revilla Gigedo, Cocos and Clipperton islands, although geographically isolated, are placed in G. a. candida. On the basis of a critical study of specimens at hand, the populations in the Pacific fit into three groups. Small birds, G. a. candida, are found in the North Pacific from the Bonins and Marianas east to Wake and the Hawaiian Chain and south to the Marshall, Phoenix, Christmas and the Fanning islands (see figure 12). Larger birds, G. a. pacifica, are found in the Central Pacific and South Pacific from the Carolines in the west southeastward through Melanesia and eastward through Samoa, to the Tuamotus and Easter to Cocos, Clipperton, and Revilla Gigedo islands. In the Southwest Pacific, at Norfolk and the Kermadec Islands, a longer-winged population occurs; it is separable as G. a. royana. The measurements of these birds are given in table 24.