Bird Research News

Vol.10 No.1

Yellow Bittern

2013.1.28.

Yoshigoi (Jpn) Ixobrychus sinensis
Breeding system:

Morphology and classification
Classification: Pelecaniformes Ardeidae
Total length:
ca. 36.5cm
Wing length: 126-146mm
Culmen length: 50.4 ± 0.2mm

Tail length:
Tarsus length:
Weight:39-49mm
40-51mm
113.1 ± 2.7g

Body weight is the mean ± SE of five adult birds (♂ 3, ♀ 2) captured in Aomori
Prefecture. The other measurements are the mean value ± SE of 40 individuals: 34
specimens in the Yamashina Institute collection as well as six individuals captured inAomori Pref.

Appearance:
Yellow Bitterns are the smallest heron breeding in Japan. They
have long toes. Since they have a tweezers-like bill with fine protrusions along the outer edge, they rarely let their captured preyslip out of the bill. The male and female are the same in body size,
but different in plumage color. A dark patch on the crown is more
conspicuous and the contrast between dark and light colors of thewings is stronger in the male. On the other hand, vertical brown
stripes of the throat is more remarkable in the female. The base ofthe bill turns red in the breeding season, which is more noticeable
in the male (Photo 1).

Yellow Bitterns winter in Southeast Asia and the Ryukyu Islands,
southernmost Japan and visit mainland Japan as a summer visitor.
They never fail to arrive in Aomori Pref., northern Japan at the endof May and leave there for the wintering grounds in the south in
September after they have finished breeding every year. But they
seem to stay as late as October in southern Japan.

Nest:
Yellow Bitterns build abasket-shaped nest above
the water by skillfully
bending emergent plants,
such as cattail, narrowleaved cattail and softstembulrush (Photo 2). They do
not seem to like reed beds
as a nesting place. Although they usually nestalone, it was reported thatthey formed a breedingcolony in the riverside areaof Saitama Pref., central
Japan. (Uchida & Matsuda
1990, Ueda 1996).

Photo 2. A nest built in the softstembulrush community. (Pond in Aomori
June 29, 2008)

Egg:
The clutch size ranged from four to eight eggs (mode = 6; Fig. 1)
in 21 nests from irrigation ponds of Aomori Pref.. Their clutch sizein Aomori Prefecure is almost the same as that of the breedingcolony of Saitama Prefecure (Uchida & Matsuda 1990). Yellow
Bitterns laid the first egg from early June to late July in 13 nests ofAomori Pref., and there was little difference in first-egg laying
date between Aomori and Saitama Prefs.

Vocalization:
Male Yellow Bitterns utter call that sounds like "Ohr, ohr" during
the breeding season. They usually call at night, but sometimes callduring the day as well.

Yellow Bitterns are year-round residents in some parts of the Indochina Peninsula, India and Sri Lanka. They are separately distributed in Seychelles and Oman as well. They are summer visitors in
the most part of China, the Korean Peninsula and mainland Japan,and winter residents in New Guinea and Borneo (Kushlan & Hancock 2005). Although Kushlan & Hancock (2005) describe themas breeders in Sakhalin, Russia and Hokkaido, northern Japan,
they are assumed to be rare at least in Hokkaido.

Habitat:Yellow Bitterns occur and breed on the edge of the water rich inemergent plants. Although they sometimes flutter over the water,
they usually move from stem to stem through emergent plants.
Since they generally spend most of the time hidden in the vegetation, they are harder to detect among waterfowl.

Life history
46 7 8 9breeding season

10

11 12

♪ http://www.bird-research.jp/1_shiryo/koe/yoshigoi_100601_watarase.mp3
Toshiaki Hirano

64
20
5

6

7

Number of eggs

Distribution:

584

Distribution and Habitat

1 2 3 4
wintering season

10

Number of cases

Photo 1. Male (left) and female (right) Yellow Bitterns in the breeding
season.

8Fig. 1. Clutch size distribution of 21
nests in the irrigation ponds ofAomori Pref.. In a nest with eighteggs, none of the eggs hatched andthe pair abandoned incubation
eventually. One of the three nests
with seven eggs was deserted
during the incubation period froman unknown cause.

Incubation and nestling periods:
All the eggs hatched 17-20 days after the last egg was laid (Uchida& Matsuda 1990). Male and female incubate the eggs in turn. During the night, however, the male or female continues incubating
alone in the study site of Aomori Pref. The female is usually responsible for the night incubation, but the male is sometimes engaged in it. The eggs do not hatch simultaneously, and therefore it
takes several days for all the eggs to hatch. Both male and female
feed the chicks, but they do not feed them at night. The chicks
grow rapidly and they rarely return to the nest about 20 days afterhatching.

Diet and foraging behavior
Yellow Bitterns lie in ambush on the stalks of emergent plants andthe pads of floating plants or walk slowly in the shallow water tocapture fish quickly from above the water. The parent birds usually feed small fish, such as loach (Misgurnus anguillicaudatus) andstone moroko (Pseudorasbora parva) to the chicks, but tree frogs
and crayfish are sometimes fed to them as well in a riverside area

♪ http://www.bird-research.jp/1_shiryo/koe/yoshigoi_090805_watarase_hirano.mp3
Toshiaki Hirano

Bird Research News

Vol.10 No.1

2013.1.28.

© Japan Bird Research Association
Sumiyoshi 1-29-9, Fuchu, Tokyo, Japanof Saitama Pref. (Ueda 1992). Yellow Bitterns nesting in an irrigation pond not only forage in the pond but also fly over the bank toadjacent paddy fields to feed. Stone morokoes are more important
prey to Yellow Bitterns foraging in a pond than loaches because
loaches do not come up to the surface of the water very often, but
stone morokoes rise to water surface for aquatic surface respiration
when the dissolved oxygen level becomes low.

Topics of ecology, behavior and conservation● Asynchronous hatching
Yellow Bittern chicks do not hatch simultaneously, which is responsible for a great difference in body size between the chicks ofthe same nest. In the study site of Aomori Pref., however, most of
the chicks successfully reach at least the stage where they are fed
outside the nest. Since Yellow Bitterns have asynchronous hatching and feeding habit in common with Grey Herons (Ardea cinerea) and Black-crowned Night Herons (Nycticorax nycticorax), it isinteresting to note that they enjoy a good breeding performance inmarked contrast to these herons. It is also characteristic of Yellow
Bitterns that their chicks grow rapidly.

● Alert behavior
It is well known that Yellow Bitterns adopt a so-called "bitternposture" where they stand erect with the bill upright to mimic areed when they are on the alert for approaching predators and humans. It is certainly difficult to detect them when they assume thisposture. Yellow Bitterns which have adopted this posture do not
run away even if people come considerably close to them probably
because they have "absolute confidence ?" in their own mimicry.
They can be captured by hand (Uchida & Matsuda 1990). Whenthe author and his co-workers carried out a study of them in irrigation ponds, they returned to the nest immediately after the study
was finished. They did not seem to mind a video camera we hadplaced right in front of the nest at all.

● Risk of extinction
Yellow Bitterns were designated as a near threatened species (NT)in the 2006 and 2012 editions of the Red List of the Ministry of theEnvironment. The decline of Yellow Bitterns breeding in Japan
can be attributed primarily to the loss of emergent plants in theirbreeding grounds resulting from the construction of concrete bankwalls and the marked decrease of small fish due to the predation ofillegally released largemouth bass (Micropterus salmoides).

● Impact of introduced fish
Small fish is the staple diet oflargemouth bass, but they also
prey on shrimps and large
aquatic insects. Even birds
were prey to them (Shimada& Fujimoto 2009). In Izunuma, Miyagi Pref. where released largemouth bass haveincreased, for instance, small Photo 3. Largemouth bass.
fish such as stone morokoes
and bitterlings have been severely depleted, which has in turngiven rise to a collapse in the population of small piscivorous
birds, such as Little Grebes (Tachybaptus ruficollis), Smews(Mergellus albellus), and Little Egrets (Egretta garzetta) (Shimadaet al. 2005). Although they did not refer to Yellow Bitterns in their

バードリサーチ生態図鑑
report, they are probably in a similarplight.
In some of the small
irrigation ponds, on
the other hand, thesurface of the water
is covered with floating-leaved
plantssuch as water chestnuts in summer,whichmarkedly
reduces the dissolved Photo 4. A "pond of oxygen deficient type"oxygen level in thewhose surface is covered with water chestwater.
Indigenousnuts.
fish species, such asstone moroko, Japanese rice fish (Oryzias spp.) and crucian carp
(Carassius spp.) can survive oxygen deficiency by rising up towater surface for aquatic surface respiration, but largemouth bass
cannot effectively adopt aquatic surface respiration and therefore
they will not increase in a pond of this type even if they are released.
Ponds which allow small fish to survive by preventing largemouthbass from increasing are of great benefit to Yellow Bitterns, LittleGrebes and Common Kingfishers (Alcedo atthis), which I call asmall piscivorous bird trio of an irrigation pond. It is a requirementfor Yellow Bitterns in particular that small fish are available justbelow the surface of the water. It is of great importance, therefore,that fish rise to the water surface for aquatic surface respiration as
well as that luxuriant water chestnuts provide footholds of locomotion even in the center of a pond. Small irrigation ponds as well aslarge lakes and marshes play a major role in conserving the faunaof bodies of water including fish.

Literature
Kushlan J.A. & J.A. Hancock. 2005. "The Herons" xv +433 pp. Oxford Univ.
Press, Oxford.
Shimada T. & Fujimoto Y. 2009. Predation of birds by Largemouth Bass. Bird
Research 5:7-9. [J+E]
Shimada T., Shindo K., Takahashi K. & Bowman A. 2005. The impact of a change
in fish community associated with Largemouth Bass increase on the waterfowl
community. Strix 23:39-50. [J+E]
Takano S. (Chief ed.). 1981. The Encyclopedia of Japanese Birds by Color Photography. 481pp. Tokai University Press, [J]Uchida H. & Matsuda T. 1990. Breeding population of Yellow Bittern Ixobrychussinensis. Jpn J Ornithol. 39:53-61. [J+E]
Ueda K. 1992. Food content of chicks of Yellow Bittern Ixobrychus sinensis. J.
Yamashina Inst. Ornithol. 24:13-17. [J+E]
Ueda K. 1996. Why do Yellow Bitterns breed colonially?: nest location selection
and reproductive success. Strix 14:55-63. [J+E]
Languages of literature cited other than English: [J] in Japanese, [J+E] in Japanesewith English summary.

AuthorYuji SAWARA

Department of Biology, Faculty of Agriculture andLife Science, Hirosaki University
I originally studied the ecology andbehavior of fish. But the study
made me realize keenly that I had
to know the behavior of piscivorous birds to understand how fish
behaved. Since then I have beeninterested in the ecology and behavior of herons, egrets and grebes.
ZUN02066@nifty.com

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