Bird Research News

Vol.6 No.12

2009.12.24.

Japanese Night Heron

Mizogoi (Jpn) Gorsachius goisagi

Morphology and classification
Classification: Pelecaniformes Ardeidae
Total length:
ca. 490mm
Culmen length: 34-41mm
Weight :470-530g

Tail length:92-116mm
Tarsus length: 64-73mm

The measurements after Kiyosu (1978), del. Hoyo et al. (1992), Uchida (1996), andDunning Jr. (2008).

Appearance:
Adult Japanese Night Herons are brightchestnut red on the head and dark brownon the back, in the wings and tail (Photo1). They are cream from the throat to thebelly with black vertical stripes, which arelonger on the throat in particular. There isno difference in plumage color between
male and female. Juveniles are darker on
the upperpart with many fine black flecks
on the head and in the wings, but someyoung birds have fine white spots in thewings (Photo 2).

Photo 1. Adult Japanese Night Heron.
Though some say
the plumage color
is quiet, I admire
the beauty.
[Takehiro Fujiwara]

Vocalization:
Japanese Night Herons make a low “Boh,Boh”call continuously from several tomore than 100 times in a bout. The callingbouts are repeated with a pause betweenthem. The call also sounds like “Pooh,Pooh” from a distance. They usually calljust after sunset and about two hours before sunrise. Although they call primarily
one or two weeks before nesting, they areoccasionally heard to call in a stopover sitewhile on migration. They sometimes callabout a week after their chicks have
hatched as well.

Breeding system:
Japanese Night Herons start to breed in early April right after theirarrival at the breeding ground. But the arrival times apparently
vary between individuals, and some of them begin to breed inJune. Since the pairs that failed the first breeding attempt generally
move to another site to nest again, some pairs fledge their chicks
in late August (Komiya & Sugita 1975). They are normally monogamous and both sexes incubate their eggs and feed their chicks.

Nest:
It was reported that Japanese Night
Herons built a nest primarily in Japanese cedar and cypress before theWorld War II (Kawaguchi 1937,Yamashina 1941). In recent years,
however, they have frequently used asa nest tree broad-leaved trees, such as
Quercus serrata, Zelkova serrata andcherry trees. In some areas, on theother hand, they prefer to nest in pine Photo 3. A Japanese Night
Heron parent showing atrees (Kurahara 1991). They build a“bittern posture” todish-shaped nest with a diameter of 40
protect the nestlings-50 cm at a fork of the horizontal
against enemies.
branches at a height of 5-15 m above
the ground (Photo 3). They do not form a breeding colony but nest
separately in a forest. The nest is usually blocked from view by thecanopy when looking at it from above, but it is easy to see fromthe ground because there is some space below it (Yamashina1941). There are a large quantity of feces with cream colored uricacid below the nest.

Egg:
Photo 2. Head of ayoung JapaneseNight Heron. It iscovered with flecks
and the bill is distinctly short.

Distribution and Habitat

The clutch size of Japanese Night Herons is 3-5 eggs. The egg iswhite and its size is 43- 49 mm by 35- 38 mm (Yamashina 1941).

Incubation and nestling periods:
The incubation and nestling periods are 20-27 and 34-37 days
respectively. Since they usually start to incubate when they lay thefirst egg, the chicks do not hatch at the same time, which is responsible for the difference in body size between the chicks.

Migration:

Distribution:
Japanese Night Herons breed up to 1000 m in the forests of Honshu, Shikoku and Kyushu (the main islands of Japan). Although
one breeding record was reported from Taiwan in the past
(BirdLife International 2001), the breeding distribution is mainly
separated from that of closely related Malayan Night Herons Gorsachius melanolophus. The observations of this species in Hokkaido (the northernmost main island) suggest a possibility of theirbreeding there as well, but specific studies are needed to confirm
it.

Habitat:Japanese Night Herons generally use a broad-leaved forest and amixed forest with coniferous and broad-leaved trees as a breeding
ground. They are frequently observed in a deciduous broad-leaved
forest, such as Quercus serrata in particular. They usually nest andforage along a mountain stream, probably because food supplies,
such as earthworms and freshwater crabs are abundant in the soil
along a stream, which is thick and wet.

Japanese Night Herons are assumed to winter primarily in thePhilippines, but they spend the winter in the other countries ofSoutheast Asia as well. During the migration season, they are also
observed in Korea, southern China and Russia (BirdLife International 2001). They seem to arrive at the breeding grounds fromMarch to June and leave them from September to November.
Since Japanese Night Herons are observed by ones or twos in thestopover sites, it is likely that they migrate singly or in a breeding
pair. It was reported, however, that more than 150 herons whichwere assumed to be Japanese Night Herons migrated in a mixed
flock of more than 1,000 herons including Black-crowned NightHerons Nycticorax nycticorax and Gray Herons Ardea cinerea(Enomoto 1927). Since details of their migration including thewintering grounds and migration routes are unknown for lack ofinformation, further studies are required.

Topics of ecology, behavior and conservation● Are Night Herons nocturnal?

Life history
12

3

4

wintering migration
period567breeding

8

9

10

11 12

migration wintering

4♪ http://www.bird-research.jp/1_shiryo/koe/mizogoi_0904_hyogo_kaji.mp3
Kyoko Kajimoto

Although Japanese Night Herons are occasionally observed in parks
and farmland, they are forest dwellers. They usually forage on a forest
floor and do not fly above the canopy very often.
They spend hours on the ground and sometimes stand still for 30
minutes or so, which makes it extremely difficult to detect them. Since
they call frequently at night, however, their presence is easy to notice.

Bird Research News

Vol.6 No.12

2009.12.24.

© Japan Bird Research Association
Sumiyoshi 1-29-9, Fuchu, Tokyo, Japan
Since they are rarely observed during the day and frequently heard atnight and, on top of that, their English name is "Night Heron", it is nowonder that they are thought "nocturnal". In fact, some literature describe them as such. But are they really so?
As a matter of fact, however, there is no report about the nighttime
feeding of Japanese Night Herons in a scientific paper. On the otherhand, many field observations show that they forage during the day. Inaddition, it was recently reported that a captive bird foraged during theday (Kawana 2009). It is an obvious fact that they call at night, but
only this fact is not sufficient to assert them to be "nocturnal". Taking
an objective view of the findings obtained until now, it may be said
that they do nothing but call at night, and are active during the day.
Yamashina (1941) reported that they left their nest site around sunset
to forage for food in the vicinity of a mountain stream and rested in aforest during the day. My observations showed that when the nestling
period progressed, the parent birds stayed in the nest in turn during theday, but both of them sometimes left the nest at night. Dr. Yamashina
probably reported on their behavior at this breeding stage.
Some bird guides describe that Japanese Night Herons also forage
during the day on gloomy days, such as on rainy and cloudy days.
When you see Japanese Night Herons foraging during the day, youmight think their daytime foraging is an exception if you have thefixed notion that they are "nocturnal". At least, however, my observations showed that they foraged in the daytime every day. Since they
are active and relatively easy to observe in the breeding period whichcoincides with the rainy season, it is no wonder that they are frequently observed to forage in the daytime on a rainy day. Contrary to popular belief, I am convinced that Japanese Night Herons are diurnal. I
must be careful not to let this assumption interfere with my future
study of the species.

● Japanese Night Herons do not necessarily like fish(%)
It is probably because Japanese
100
people are familiar with fishingherons such as Black-crowned
shellfishNight Herons and egrets that 80
they tend to associate herons
spiderwith fish catching behavior. 60
However, the diet of Japaneseriver crab
Night Herons consists primarily 40of freshwater crabs, land snails
insectand insects as well as soil ani20
mals such as earthworms (Fig.
1, Kawakami et al. 2005). Japa0
nese Night Herons have a dis東京A 東京B
埼玉広島
tinctly shorter and thicker billFig.
1.
Resultofpellet
analysis
inthan other herons. It is assumed
four nests. Earthworms, the staple
that piscivorous birds generally
food of Japanese Night Herons,
have a long bill so as to catch
were not detected by pellet analyelusive fish quickly from a dissis due to their digestibility. Mosttance. On the other hand, the billof the shellfish were land snails.
of Japanese Night Herons is
Modified from (Kawakami et al.
morphologically suitable to2005).
capture soil animals, not fish.
Some bird guides count fish as one of the food items of Japanese Night
Herons, but they would rarely, if ever, capture fish.

● The day Japanese Night Herons will become extinct
It is suggested that Japanese Night Herons have been declining acrossthe country since the mid-20th century (Kawakami & Higuchi 2003).
For instance, one pair nested along each mountain stream around 1950
in Kunado, Yamanashi Prefecture, central Japan, but almost no Japanese Night Herons nested in this area around 1990 (Hyuga 1949,1993). The frequency in the observation of this species has recentlydecreased drastically in other areas, such as Miyakejima Island of theIzu Islands as well. It is reported that many species of summer residents have had a tendency to decline since the late 20th century, whichis assumed to result from the deforestation in their wintering grounds

バードリサーチ生態図鑑of Southeast Asia. In the Philippines, the major wintering ground ofJapanese Night Herons, the forest was dramatically reduced in thelatter half of the 20th century, which is probably responsible for their
population decline. In Japan, on the other hand, a high proportion ofbroad-leaved forests were replaced with plantations of Japanese cedar
and cypress in this period. I consider that the reduction of their suitablebreeding habitat also played a role in their population decline.

● Is a thousand herons true?
The population of Japanese Night Herons have not been thoroughly
studied yet. Although some authors estimate the population of Japanese Night Herons at less than 1,000 birds, no scientific evidence isshown for the estimation. Assuming that 1,000 birds (or 500 pairs) areengaged in breeding and that the forest area below1,000 m (asl.)amounts to 200,000 km2 in Honshu, Kyushu and Shikoku (the major
breeding grounds of Japan), the breeding density is one pair per 400
km2 (20 km by 20 km). It is true that Japanese Night Herons are on thedecline and therefore their population density is extremely low, but
this estimation is probably too conservative, judging from my personal
study experience. Rarity may accelerate conservation efforts, but theunderestimation of a population carries a risk on the other hand. Iffurther studies reveal that the actual number of the species is not anestimated 1,000 birds but 10,000 birds, it looks as if the population has
increased by a factor of ten. Therefore, population underestimation isnot recommended.

Literature
BirdLife International. 2001. Threatened birds of Asia: the Birdlife International
Red Data Book Part A. BirdLife International, Cambridge.
del Hoyo J., Elliott A. & Sargatal J. 1992. Handbook of the Birds of the World,
Volume 1. Lynx Editions, Barcelona.
Dunning J.B. Jr. 2008. Avian Body Masses. CRC Press, Boca Raton.
Enomoto Y. 1927. Migratory herons. Wildlife Collection Report 3 (5):13-15. [J]Hyuga F. 1949. Observation of Japanese Night Herons. Yacho 14(9) :19-22. [J]Hyuga F. 1993. Joy to host summer bird, now. In: Endo K. (ed.) The song of summer birds now. Sanseido, Tokyo. [J]Kawaguchi M. 1937. Ecological notes on Japanese birds. Sorin Shobo, Tokyo. [J]Kawakami K., Uchida H. & Fujita M. 2005. Diet of the Japanese Night Heron
Gorsachius goisagi in Japan. Ornithol. Sci. 4:173-177.
Kawakami K. & Higuchi H. 2003. Population trend estimation of three threatenedbird species in Japanese rural forests: Japanese Night Heron Gorsachius goisagi,
Goshawk Accipiter gentilis and Grey-faced Buzzard Butastur indicus. J. Yamashina Inst. Ornithol. 35:19-29.
Kawana K. 2009. Foraging behavior of Japanese Night Herons in captivity in the
non-breeding season. J. Yamashina Inst. Ornithol. 41:62-64. [J+E]
Kiyosu Y. 1978. Great pictorial guide to Japanese Birds III. Kodansha, Tokyo. [J]Komiya T & Sugita H. 1975. Observation of breeding Japanese Night Herons in thewild. Animals and Zoos 24:24. [J]Kurahara K. 1991. Birds in the Watarase River. Yamaibo-sha, Ashikaga. [J]Uchida H. 1996. Japanese Night Herons. In Higuchi H. et al. (eds.) The encyclopedia of animals in Japan. Birds 1. Heibonsha, Tokyo. [J]Yamashina Y. 1941. Birds of Japan and its ecology, the second volume. Iwanami
Shoten, Tokyo. [J]Languages of literature cited other than English: [J] in Japanese, [J+E] in Japanesewith English summary.

AuthorKazuto KAWAKAMI Forestry and Forest Products Research InstituteTen years have passed since I started to study Japanese Night Herons. I
am getting obsessed with the ideathat they bear a striking resemblanceto "Kyoro-chan", a bird character of
Chocoball produced by Morinaga &Company Ltd. I would like Morinaga to make a box of chocolate balls
featuring Japanese Night Herons aswell.
kazzto@ffpri.affrc.go.jp
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