Bird Research News

Vol.3 No.9

2006. 9.13.

Izu Islands Thrush

Akakokko (Jpn) Turdus celaenops

Morphology and classification
Classification: Passeriformes Muscicapidae
Total length:
Wingspan:
Tail length:
Tarsus length:
Weight:Approx. 230mm♂ 112.0-124.0mm♂ 82.0-100.8mm♂ 31.9-35.8mm♂ 73.7-81.3g

Culmen length: 20-23mm♀ 108.0-117.0mm♀ 74.0-89.0mm♀ 32.5-33.7mm♀ 65.9-93.1g

Total length and bill length are quoted from Takano (1990), and Kiyosu (1978), respectively. Other measurements are taken in the banding study at Miyakejima Island.

Appearance:
Male is black from the head toward the chest, and orange on theabdomen and flank. The bill and the area around the eye are yellow. The back, shoulders and wing coverts are dark brown. Female
is somewhat paler than male. Individuals of the Tokara Islands,
southern Japan are darker than thoseof the Izu Islands, Tokyo.

Vocalization:
Male sings at dawn (about 30
minutes before sunrise) in a guttural
voice that sounds like "Kyurrr,
Chotts" during the first half of thebreeding season. In the second halfof the breeding season, however, hesings during the daytime as well. Thecalls sound like, "Tweet", or "Chat,Chat, Chat".

Photo 1. Male Izu Islands
Thrush. [Photo by NakataKazuma]

Distribution and Habitat
Distribution:
Izu Islands Thrushes occur in the Tokara and the Izu Islands. They
are also observed in Yakushima Island and the Danjo Islands,western Japan. In the Izu Islands, they are distributed from Oshima
to Aogashima Islands, with their populations larger in the southern
islands than the northern ones. In the Tokara Islands, they inhabit
Nakanoshima, Kuchinoshima, Akusekijima, Tairajima andTakarajima Islands, but the population density is lower than that ofthe Izu Islands. In addition, there are wintering records from otherareas, such as Shizuoka (Yamashina Inst. Ornithol. 1996), Chiba,
Mie and Wakayama Prefectures.

Habitat:
The species occurs in a variety of habitats from natural to secondary forests of evergreen and summer green broad-leaved trees. The
population density tends to be high in an area where the shrub
layer of an evergreen broad-leaved forest is not well developed(Higuchi 1992). It is not only designated as a national monument
but also listed as an endangered species in the Red Data Book of
the Ministry of the Environment. This endemic thrush faces thereduction of its habitat due to the volcanic activity on Miyakejima
Island in particular.

Life history
12

3

45

They build a bowl-shaped nest in atree at a height of 0.5-6m above theground (Kiyosu 1978), solidifying
fine tree roots, grass stems, moss andother materials with soil. The nest
has an outer diameter of 13-16.5cm,
an inner diameter of 8-10cm, a depthof 5.5-7cm and a height of 9.5-13cm.

Egg:

Photo 2. Eggs and nest of Izu
Islands Thrushes. [Photoby Tanaka Toshihiko]

Clutch size is usually three or four eggs, ranging from two to fiveeggs. The egg has a long diameter of approximately 29mm by 21mm. The egg has red brown flecks on a blue background.

Incubation and nestling periods and fledging rate:
Incubation period is unknown.
Nestling period is about 14 days.
Due to the weasel introduction
for rat control in Miyakejima
Island of the Izu Islands, thefledging rate drastically declined
from 85% (Higuchi and Koike1977) and 71-78% (Higuchi1981) to about 7.3% (Takagi &Higuchi 1992).

Photo 3. Izu Islands Thrush
chicks. [Photo by TanakaToshihiko]

Diet and foraging behavior
This thrush feeds primarily at a forest edge in the early morningand in the evening. Its diet consists mainly of earthworms, crustaceans, centipedes and insects, but also includes fruits of mulberry,raspberry, Machilus, cherry, hackberry, Stachyurus and Polygonum seasonally. The result of video recording showed that earthworms accounted for about 70% of the food fed to the chicks andthe rest included nuts, centipedes, insects and others.

Topics of ecology, behavior and conservation● On the origin of this endemic thrush
There are two theories on the origin of Izu Islands Thrushes: (1)they are a relatively new species closely related to Brown Thrushes, and (2) they are a relict species closely related to Blackbreasted Thrushes Turdus dissimilis in southern China. The plumage comparison, such as the pattern of undertail coverts and thecolor of wing coverts suggests that Izu Islands Thrushes are closely related to Brown Thrushes because Izu Islands Thrushes sharecommon features with Brown Thrush superspecies (T. chrysolaus,T. pallidus and T. feae), but Black-breasted Thrushes have characteristics in common with T. hortulorum. It is reasonable to assume,
therefore, that Izu Islands Thrushes and Black-breasted Thrushes
belong to distinct clades of genus Turdus (Kajita et al. 1998).

● Song difference between the island populations
6breeding season

78910

11 12

non-breeding season

Breeding system:
The breeding season is between March to July. Izu Islands Thrushes are monogamous, but they live singly in the non-breeding season. Both male and female are responsible for incubation and feeding of the chicks.

42Nest:The song of the Izu Islands Thrushes is similar to that of Brown
Thrushes, but it is shorter and more guttural. The song of Miyakejima Island population consists of two syllables, as shown in Fig 1.
The song of the population of Mikurajima Island which is locatedabout 18 km south of Miyakejima Island is similar to that ofMiyakejima population. However, the song of the population ofHachijojima Island which lies approximately 90 km south ofMiyakejima Island is made up of a single syllable (Fig. 2), andBird Research News

Vol.3 No.9

2006. 9.13.

© Japan Bird Research Association
Sumiyoshi 1-29-9, Fuchu, Tokyo, Japan(KHz)

song

10Syllable

バードリサーチ生態図鑑

5sounds entirely different
from that of Miyakejima
population. In addition,
the songs of the populations of Aogashima Island which is south ofHachijojima Island andNakanoshima Island ofthe Tokara Islands arecomposed of more syllables (Fig. 2).

510 (S)
Number of Syllables

0Fig. 1. Sonogram of aIzu Islands
Thrushsong(Miyakejima).

43210Miyake Mikura Hachijo
jima
jima
jima

Aoga Nakano
shima shima

Fig. 2. Syllable number of songs in different islands● Reduced breeding success rate due to weasel introduction
Weasels were introduced to Miyakejima Island in the 1970s and1980s to reduce rat damage to crops. In particular, Izu Islands
Thrushes declined drastically due to the release of weasels around1982. The eggs and chicks in a nest are prone to the predation byweasels that can climb trees. The population density of this thrush
dwindled to about 1/3 and the reproductive success rate dropped
from 80% to about 7 percent (Takagi and Higuchi 1992). Since
weasels were introduced to other islands of the Izu Islands, such as
Hachijojima, Toshima and Aogashima Islands in former times, it isassumed that Izu Islands Thrushes also declined on these islands
due to the predation of introduced weasels. It is absolutely essential to control weasels to recover the populations of Izu Islands
Thrushes on these islands.

● Impact of volcanic activity on the habitat
Volcanic ash, mudflow andgas resulting from the volcanic eruption of June 2000
have affected about 60%of the forest area in Miyakejima Island, the major habitat of Izu Islands Thrushes.
In the early volcanic activity, volcanic ash emitted
from the crater of Mt. Oyama piled up more than 1m Photo 4. Forest of eastern Miyakejima
in the area above 400 m
Island where most trees have withabove sea level and denuded
ered.
most of the forest. In theeastern area downwind from the crater, most trees have died due tovolcanic gas and Izu Islands Thrushes have disappeared (Photo 4).
The effects of 2000 eruption on small ground-dwelling animals
were investigated in 2002. The result showed that earthworms
were confirmed only in the "locations without ash nor gas damage" among the "locations with ash damage", "locations with gasdamage", "locations with both ash and gas damage" and “locations
without ash nor gas damage”.
In June 2006, the study of soil animals was carried out in conjunction with the census of birds in similar sites (four points, two samples each). Soil animals were not detected in the "locations withboth ash and gas damage". Only insects were confirmed in the"locations with ash damage", while earthworms, beetles, pill bugs
and springtails were confirmed in the "locations with gas damage"and earthworms, beetles and Geophilids occurred in the "locations

without ash nor gas damage". Izu Islands Thrushes were not detected in the "locations with both ash and gas damage". Although
the habitability of Izu Islands Thrushes must be measured in termsof not only food but also a shelter and a nesting site, it is assumed
that the volcanic eruption still have a negative effect on their range
because no earthworms that are their staple food occur in the"locations with volcanic ash damage".
Mt. Oyama emitted a record-level volume of volcanic gas, whichamounted to 20000-50000 tons per day at the peak period(December 2000). In 2006,
however, the amount ofvolcanic gas was reduced
to about 1/20 (1000-3000
tons per day). As the volcanic gas decreases, theforests at the foot of Mt.
Oyama have been gradually recovering (Photo 5). I
hope that the forests andsoil will recover soon even Photo 5. Nature is recovering. Miscanat high altitudes and Izuthus and Alnus have started to penetrate even into areas covered with
Islands Thrushes will remud flow or stricken by volcanic
turn to their former habitats
gas.
of this island.

Literature
Higuchi H. & Koike S. 1977. Breeding success of birds after the weasel introduction in Miyakejima. Yaseiseibutsuhogo :81-88. [J]Higuchi H. 1981. Effects of the weasel introduction on islands on wild birds: bird
habitat survey report. Tokyo Metropolitan Labor Economic Affairs Bureau,Tokyo. [J]Higuchi H. 1992. Survey Report on the ecology and conservation of birds. Tokyo
Metropolitan Bureau of Labor and Economy :27-64. [J]Kajita M., Ohta N. & Kakizawa R. 1998. Phylogenetic study of birds of the Izu
Islands (on Bush Warbler and Izu Islands Thrush). Report of eco-tourism andbirds of islands: 52. [J]Kiyosu Y. 1978. Encyclopedia of Japanese birds. Kodansha, Tokyo. [J]Takagi M. & Higuchi H. 1992. Habitat preference of Izu Islands Thrush Turdus
celaenops and the impact of weasel introduction on Miyakejima Island. Strix
11:47-57. [J+E]
Takano S. 1990. Field guide of Birds of Japan. Wild Bird Society of Japan, Tokyo.
[J]
Yamashina Institute for Ornithology. 1996. Bird Banding Research Report (1996):
17. Abiko. [J]Languages of literature cited other than English: [J] in Japanese, [J+E] in Japanesewith English summary.

AuthorYutaka YAMAMOTO Wild Bird Society of Japan
Thirteen years have passed since
I first saw Izu Islands Thrushes.
It is a familiar bird that everyone
knows in the Izu Islands, but it
does not mean that the study ofits ecology and the solution to
the conservation issues has
advanced enough. Making useof this opportunity to organize
the information on this endemic
thrush, I would like to makesteps toward solution eventhough they are small.
y-yamamoto@wbsj.org

The author guiding visitors inMiyakejima Island (Photo left).

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