_Conopoderas luscinia nijoi_ Yamashina, Tori, 10, 1940, p. 674.
(Type locality, Agiguan.)
_Conopoderas luscinia nijoi_ Hand-list Japanese Birds, 3d ed.,
1942, p. 196 (Agiguan).
_Acrocephalus luscinia luscinia_ Mayr (part), Birds Southwest
Pacific, 1945, p. 294 (Agiguan).
_Geographic range._--Micronesia: Mariana Islands--Agiguan.
_Characters._--Adult: Resembles _A. l. luscinia_, but with shorter
bill. Yamashina (1940:674) describes the birds as, "upper parts
much less rusty in colour and the flanks and bellies are darker
and more brownish than those of the specimens from Almagan and
Saipan."
_Measurements._--Yamashina (1940:674) gives the measurements of
five adult birds from Agiguan as: exposed culmen 27-29, bill from
nostril 17.0-20.0; as compared with 27 adult birds from Almagan
and Saipan as: exposed culmen 30-34, bill from nostril 21.2-24.5.
_Remarks._--No specimens have been examined by me. The island of Agiguan
is a very small one lying offshore from Tinian and not far from Saipan,
where _A. l. luscinia_ occurs. _A. l. nijoi_ is given tentative
recognition, on the basis of the measurements of the five adult
specimens given by Yamashina. These indicate that the population has a
distinctly shorter bill.
_Evolutionary history of Acrocephalus luscinia._--The species of
_Acrocephalus_ in Micronesia and Polynesia have received several
taxonomic treatments. In regard to the Micronesian forms, Quoy and
Gaimard called the population at Guam _Thryothorus_ while Kittlitz
called the population in the Carolines, _Sylvia_. Evidently to emphasize
the distinctness of these two birds, Reichenbach in 1850 renamed the
bird in the Marianas as _Hybristes_ and the bird in the Carolines as
_Eparnetes_. The birds were later placed in the genus, _Tatare_, by
Hartlaub, Gray, Sharpe and other workers. Gray also used the name,
_Calamodyta_, for the bird in the Carolines. The generic term,
_Calamoherpe_, was employed also by a number of workers for the Caroline
population. Sharpe (1883:525) placed the reed-warblers in the family
Timelidae and retained the name, _Tatare_, for the Micronesian and
Polynesian forms. In distinguishing _Acrocephalus_ from _Tatare_ he has
the following to say of _Acrocephalus_: "besides having a much shorter
bill, possesses a very much more pointed wing, the distance between the
primaries and the secondaries being much more than the length of the
hind toe and claw; whereas in _Tatare_ the wing is much more obtuse, and
the distance between the primaries and the secondaries is less than the
length of the hind toe and claw." More recent authors have followed
Sharpe using the generic name, _Conopoderas_ (= _Tatare_, old name
preoccupied). However, Tristram (1883:38-46) regarded the separation of
these oceanic forms from _Acrocephalus_ as a taxonomic error. He said
that this is "one of the very few links (the others being the solitary
_Hirundo tahitica_ and the Merulae) between the avifauna of Oceania and
our own; and it has a much wider range east and west than either of the
other links, extending from the Carolines in the east to the Marquesas
in the west." Mayr has pointed out (orally to the writer) that the
separation of the Oceanic reed-warblers from _Acrocephalus_ is an
unnatural one, although it is perfectly true that the extreme members
(_A. caffra_ and _A. l. luscinia_) have a very long bill, but forms with
shorter bills like _A. l. syrinx_ point to the close affinity between
the continental species and these insular birds. This has also been
noted by Hartert (1898:58). Mayr (_in litt._) comments that "There is no
difference between _Acrocephalus_ and _Conopoderas_ in regard to the
wing formula, provided that we compare the Polynesian species with the
tropical forms of _Acrocephalus_ (such as _toxopei_ and _cervinus_). The
character mentioned by Sharpe is very artificial and merely indicates
the difference in the wing between a migrant of the temperate zone and a
resident of the tropics. There is no denying that some of the warblers
of eastern Polynesia are no longer reed-warblers but have become
dwellers of trees and bushes. However, this same tendency prevails among
some of the unquestionable species of _Acrocephalus_ (_scirpaceus_ and
_palustris_) and at any rate a slight change in habits is not sufficient
for generic separation." Earlier, Mayr (1942b:169) used _Conopoderas_ as
one of the several genera that is based on "morphologically distinct
geographic forms." The degree of modification that has occurred in these
oceanic reed-warblers, would, if the birds were in a continental area,
undoubtedly be considered worthy of specific or even generic rank by
some authors; however, as Mayr (1942b:162) points out, "the majority of
well-isolated subspecies have all the characters of good species and are
indeed considered to be such by the more conservative systematists."
Owing to their differentiation, the Micronesian and Polynesian
reed-warblers might not be considered by some ornithologists as
belonging to a single superspecies; however, all evidence seems to point
to the origin of this group by a single invasion from Asia."
Tristram (1883:41) was the first worker to recognize the relationship of
the Micronesian and Polynesian reed-warblers to the continental forms,
when he placed them within the genus _Acrocephalus_. Rothschild (1893:2)
further stated, "_Tatare_ cannot be separated generically from
_Acrocephalus_." In discussing the status of the Hawaiian species, _A.
familiaris_, Hartert (1898:58) also follows this treatment. Bryan
(1941:187) also comments on the relationship of the "miller" birds of
Laysan and Nihoa to species at Guam, Christmas and other islands of the
Pacific.
The reed-warblers of Polynesia and Micronesia represent an ancient
invasion from Asia. The continental form, _Acrocephalus arundinaceous_,
is apparently closest to the ancestral stock of these oceanic birds.
This species resembles the oceanic populations in size, general
coloring, shape of bill, and wing and tail structure. Some of the
continental races of this species have a shorter first primary which is
similar to that in the oceanic forms. How rapid the spread was of the
reed warbler through the large insular area that it now occupies is
unknown. _A. syrinx_ of Micronesia has a shortened wing and some
populations have a long bill. Species in Polynesia have stronger wings
than the one in Micronesia, but have become differentiated in other
ways, as, for example, by the long bill of _A. caffra_ and the small
size of _A. aequinoctialis_. In addition, call notes have become varied,
as noted by Chapin (in Mayr, 1942b:54). Also certain of the
reed-warblers have become bush and tree-living birds. The Hawaiian birds
are reduced in size and have become tree-living in a manner similar to
that of other Polynesian species. These modifications of the
reed-warblers of the Oceanic area appear, according to Murphy and
Mathews (1929), to indicate their long-time residence in the islands, as
compared with subspecies of _A. arundinaceous_ that are found in
Melanesia. The latter birds, which are not ancestral to the Polynesian
birds, resemble closely their Asiatic ancestors and have also retained
their swamp-living habits. This would seem to indicate that the birds in
Melanesia may be of more recent occurrence. Stresemann (1939b:324)
presents a map of the distribution of _A. arundinaceous_ in southeastern
Asia and adjacent islands. The original stock came from a point in
China, north of Indochina, spreading to the Philippines and to Celebes,
from where it reached the Solomons and New Guinea via the Lesser Sundas
and Australia.
[Illustration: FIG. 15. Geographic distribution of _Acrocephalus_ in the
Pacific area and routes of its dispersal. (1) _A. arundinaceus_; (2) _A.
luscinia_; (3) ranges of _A. atypha_, _A. caffra_, and _A. vaughani_;
(4) _A. aequinoctialis_; (5) _A. familiaris_.]
The path of invasion of Oceania by the reed-warbler is pictured in
figure 15. Probably the birds became established in Micronesia by an
invasion from the Bonins, where _A. arundinaceus orientalis_ is known to
occur today. From the Marianas and Carolines, the birds spread to
Polynesia; _A. l. rehsei_ of the Gilbert Islands (Nauru) might well be a
connecting link. Possibly, the Hawaiian birds came as a separate
invasion _via_ the Volcano and Bonin islands or through the Micronesia
Chain, or through the Line and Christmas islands from the south. It
seems evident, however, that owing to their geographic proximity and
comparative structural similarity, the species in Hawaii is closest to
_A. luscinia_ of Micronesia. The absence of reed-warblers from the
western Carolines and Palaus seems to reduce the possibility of an
invasion from the Philippine region. However, reed-warblers are absent
from the Marshall and the northern Gilbert islands, where there is
undoubtedly suitable habitat for their occurrence. Possibly these
islands were once occupied by the birds but they were eliminated by
natural causes or by man and his land uses.
=Rhipidura rufifrons uraniae= Oustalet