_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.
[Illustration: FIG. 12. Geographic distribution of _Gygis alba_ in the
Pacific area. (1) _G. a. candida_; (2) _G. a. pacifica;_ (3) _G. a.
microrhyncha;_ (4) _G. a. royana_.]
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.