29

Invasive Birds in Japan

Kazuhiro EGUCHI1 and Hitoha E. AMANO2*
1

Department of Biology, Faculty of Science, Kyushu University
Fukuoka 812-8581, Japan.
E-mail: kegucscb@mbox.nc.kyushu-u.ac.jp
2
Graduate School and Cultural Studies, Kyushu University,
Ropponmatsu, Fukuoka 810-8560, Japan*
Present address: WWF Japan, 3-1-14 Shiba, Minato-ku, Tokyo 105-0014, Japan

Abstract
Introduction of exotic birds has been increasing throughout the world. We conducted inquiry
investigations in order to reveal the spread of introduced bird species in Japan. Forty-three exotic
species are known to breed or are regarded as breeding. The Feral Pigeon (Columba livia), Chinese
Bamboo Partridge (Bambusicola thoracica), Red-billed Leiothrix (Leiothrix lutea), Budgerigar(Melopsittacus undulatus) and Red Avadavat (Amandava amandava) are widely distributed. The
Rose-ringed Parakeet (Psittacula krameri manillensis), Common Peafowl (Pavo cristatus),
Light-vented Bulbul (Pycnonotus sinensis), Melodious Laughing Thrush (Garrulax canorus), Crested
Myna (Acridotheres cristatellus) and Black-billed Magpie (Pica pica sericea) are locally common,though they have limited ranges. Sightings of other species were sporadic and regionally limited.
Most of introduced birds in Japan are escaped caged birds. About two thirds are from tropical regions,and show diet generalism as omnivores, granivores and frugivorous or herbivorous insectivores.
Light-vented Bulbuls, Feral Pigeons and Common Peafowl exerted crop damage. Although adverse
effects by exotic birds are not prominent yet, there is great concern that exotic birds will become a threat
to native biota and habitats in future.
Key words: introduced birds, invasive birds, naturalization

1. Introduction
Disturbance of native biota and devastation of
local ecosystems are major adverse effects caused byinvasive species. Nile perches prey upon native
fishes and have changed the fauna of Lake Victoria
remarkably (Hunter, 1996). Goats, cattle and pigs
destroy the habitat of islands (Pimentel et al., 2000).
Introduced carnivorous mammals, including rats, cats,mongooses and weasels, have caused local extinctionof native bird species in the Hawaiian Islands, New
Zealand and other oceanic islands (Pimentel et al.,
2000; Levy, 2003). Many birds have also been transported by humans to new areas of the world. The
ecological effects of introduced birds on native biota
and ecosystems are known, and some are harmful(Long, 1981; Lever, 1987).
The introduction process is composed of threephases; arrival or entry, establishment and integration(Vermeij, 1996). A population may increase after
establishment and proceed into the integration phase,where the species interacts with the recipient commuGlobal Environmental Research8(1)/2004: 29-39
printed in Japan

nity and, eventually, is integrated into it. Increasing
population of exotic species can exert a negative
influence on local ecosystems and native biota. In
general, biotic invasion causes disturbance or destruction of habitats and ecosystems, and decline or
extinction of native biota (Long, 1981; Lever, 1987).
Risk assessment is a tool for decision-making onwhether or not a species is to be introduced orimported and for establishing eradication or regulation
programmes against established invasive species(Wittenberg & Cock, 2001). It assesses the rate ofentry, likelihood of successful introduction, rate andextent of spread, and economic and ecological impact.
However, because no effective methods of assessing
the risk of entry, establishment and spread of potential
invasive species have been established yet, risk
assessment schemes have been practiced in only alimited number of countries (Wittenberg & Cock,2003). In Japan, the Ministry of Environment formulated a new Biodiversity Strategy in 2002 (http://
www.biodic.go.jp/nbsap.html).
It includes threemain actions in response to biotic introduction:©2004 AIRIES

30

K. EGUCHI and H. E. AMANO

prevention, assessment and management (regulationand eradication). Because most of the introduced
birds and mammals in Japan are escaped captive
animals, a legal barrier against the import of animals
is necessary. Legal regulation depends on accurate
evaluation of invasibility and risk of ecological andeconomic damage to the recipient ecosystem andbiotic community.
In Japan, there have been no nationwide studies on
the spread of invasive birds so far. In this study, we
carried out a nationwide inquiry of the distribution of
exotic birds and their impact on native biota andhabitats. Most of these species were recently introduced and the number of founders is small, probably
less than ten pairs. The aim of this study is toattempt to assess the likelihood of spread and risk of
impact of these invasive species on native biota andecosystems. Diamond and Case (1986) listed mechanisms of effects on biota by avian invasion: habitat
destruction, predation, brood parasitism, disease,hybridization and competition. In this paper, we will
review various influences of introduced birds onnative biota and ecosystems overseas in regions where
they have been introduced first and then discusspossible risks of impact in Japan.

2. Methods
Exotic species known to breed in Japan are listed
in Table 1.
Species are nominated based on
information collected by inquiries, supplemented withdata from literature; Iozawa (2000), OrnithologicalSociety of Japan (2000) and Tanaka & Takehara(2003). Investigations were conducted in 2000 and2001 by e-mail to members of mailing lists (“jeconet”
and “banding”) and the Ornithological Society ofJapan, inquiries on journals of the Wild Bird Societyof Japan (WBSJ), Sapporo Branch of WBSJ,
Kitakyushu Branch of WBSJ and Japanese Society for
Preservation of Birds, and personal inquiries.
Inquiry data were collected from 34 of Japan’s 47
prefectures. Information is not evenly distributed
throughout the country, but biased to the western half
(Fig. 1a). In total, information on 564 cases wascollected. While a lot of information, 351 cases, wascollected from three prefectures, Nagano, Hyogo andSaitama, there were fewer than ten cases each from 22
prefectures. Therefore, we could show only roughtendencies. The extent of spread for each species
was shown at the prefectural level (Table 1, Fig. 1).
We included only species in which breeding issuspected according to evidence such as breeding
behavior, existence of juveniles, or grouping of adultmales and females during the putative breeding season.
Only species in which sightings at more than one
place were obtained were included. We omitted thedata if an individual was suspected to be a vagrant.
In Table 1, we also included the mode of introduction, habitat type in Japan, clutch size, diet, habitat

type and climate type of the original region, impact of
introduced populations on native biota and habitats inand outside of Japan. Information on Japan was
based on inquiry data, the Yamashina Institute for
Ornithology (1979), Kuroda & Komiya (1987), Kinjo
et al. (1987), Kinjo (1998), Eguchi & Takeishi (1997)
and Tanaka & Takehara (2003). Information on
original habitats and regions of introduction overseas
were from Long (1981), Lever (1987), del Hoyo et al.
(1992, 1997), Robson (2000) and Zheng & Zhang(2002).

3. Invasive Species in Japan
In total, 43 exotic species are known to breed inJapan (Table 1).
The Muscovy Duck Cairina
moschata and Feral Pigeon (Columba livia) are feral
escapees of domesticated races introduced as food or
racing pigeons. The Chinese Bamboo Partridge(Bambusicola thoracica) and Ring-necked Pheasant(Phasianus colchicus karpowi) were introduced andreleased for hunting more than 70 years ago (Kiyosu,
1978; Kuroda & Komiya, 1987). The Black-billed
Magpie (Pica pica sericea) was introduced andreleased in northern Kyushu in the late 16th century,and it is very common there now (Eguchi & Kubo,
1992). All other species are escaped caged birds
imported as pets. Therefore, Psittidae, Timaliidae,
Estrilidae, Ploceidae and Sturnidae accounted formore than half of the species listed in Table 1.
Feral Pigeons have spread throughout the country
because of a long history of introduction, having been
introduced at least 600 years ago (Yamashina Institutefor Ornithology, 1979). For Chinese Bamboo Partridges, we obtained sight records from 17 prefectures.
However, a nationwide census conducted by the
Agency of Environment (1981) showed that thisspecies was distributed in about two thirds of thecountry, mainly in the western half of Japan. Besides
these two species, the Budgerigar (Melopsittacus
undulatus), Red-billed Leiothrix (Leiothrix lutea) andRed Avadavat (Amandava amandava) have spread in
more than 10 prefectures (Table 1, Fig. 1b – 1d).
Budgerigars and Red Avadavats are concentrated in
the Kanto region and scattered in southwestern Japan(Fig. 1b, 1c).
Distribution of the Red-billed
Leiothrix are concentrated around the Kanto region,
Kinki region and Kyushu (Fig. 1d), in particular they
inhabit all of Kyushu in high abundance (Eguchi &Amano, 2000).
Most introduced land birds inhabit habitats disturbed by humans such as croplands, riparian grasslands and town areas, which correspond to their
original habitats (Table 1). Exceptions include four
species of Timaliidae, the White-browed Laughing
Thrush (Garrulax sannio), Spectacled Laughing Thrush(G. perspicullatus), Melodious Laughing Thrush (G.
canorus) and Red-billed Leiothrix, which inhabit
forests. Particularly, latter two species have been

Invasive Birds in Japan

31

Table 1 Exotic birds breeding in Japan.
Overseas
introduced
populations2

Original populations2

In JapanSpeciesModeof entry

No. of cases/
prefectures*

Habitats

Impactreported1

Habitat type

Climatetype

Diet

Clutchsize

Impact reported

EE
F

16/7(2)
4/2
7/5

waterswaterswaters

n/k
n/k
n/k

waterswaterswaterstemp
temp
wet trop

omni
omni
herb

3-7
6-15
8-15

crop/comp
n/k
---

E

3/1

watersn/kwaterswet trop

omni

3-4

---

I
I
E
E3

27/17
11/3
6/3
---/---(1)3forfor/crop
for/rip
for/crop3

n/khybridn/kcrop/pred3

for
for/crop
for/sav/crop
sav/crop

temp
temp
temp
wet trop

herb/ins
herb/ins
herb/ins
omni

3-7
8-14
12-18
3-8

n/k
crop
n/k
n/k

F

---/coutrywide

town

crop/disrocky

temp/tropherb

2

crop/comparid temp gran/ins
dry trop
gran
wet trop
frug
wet trop frug/granwet trop
herb

4-8
1-11
3-4
3-4
3-4

crop
crop
crop/dis/comp
crop
crop
crop
----n/kcompcrop/dis/comp
n/k
n/k
n/k
crop
crop
--crop--crop/comp
crop
n/k
n/k
n/k
crop
------crop/dis/pred/comp--crop/hybrid
crop/compn/k---Anseriformes
Cygnus olor
Aix sponsa
Cairina moschata
Ciconiformes
Mycteria leucocephala
Galliformes
Bambusicola thoracica
Phasianus colchicus
Colinus virginianus
Pavo cristatus3
Columbiformes
Columba livia
Psittiformes
Melopsittacus undulatus
Myiopsitta monachus
Psittacula eupatria4
Psittacula krameri
Psittacula alexandri
Passeriformes
Pycnonotus jocosus
Pycnonotus sinensis5
Garrulax perspicillatus
Garrulax sannio
Garrulax canorus
Leiothrix lutea
Paroaria coronata
Estrilda troglodytes
Estrilda melpoda4
Lonchura punctulata
Lonchura malacca
Lonchura atricapilla
Lonchura striata6
Lonchura maja
Padda oryzivora
Amandava amandava
Vidua paradisaea
Vidua macroura
Euplectes afer6
Euplectes orix
Ploceus manyar
Ploceus intermedius4
Sturnus contra
Acridotheres tristis

E
E
E4
E
E

24/9(3)
2/2---/---(1)4
17/6
3/1(1)

town/rip
town
town4
town
town

n/k
n/k
n/k
n/k
n/k

sav
sav
for
for/crop
for

E
E5
E
E
E
E
E
E
E4
E
E
E
E6
E
E
E
E
E
E6
E
E
E4
E
E

2/2
---/---(1)5
6/3
2/2
36/6(3)78/21(1)
4/3(1)
3/2(1)---/---(1)4
8/3
10/4(1)
8/6(3)---/---(1) 6
13/5(1)
9/2(2)
63/12(2)2/2(1)
4/3---/---(1) 6
4/4
2/2---/---(1)4
3/1(1)
4/1(2)

town
town/crop5
for
for
for
for
rip/crop
rip/crop/town
rip/crop4
rip
rip
rip/crop
crop6
rip
rip/crop
rip
town
rip
rip/crop6
rip/crop
rip
rip/crop4
town
town

n/k
crop5
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k
n/k

sav/forfor/crop
for/crop
for
for
for
for
sav/rip
sav/crop
sav/crop
sav/crop/rip
sav/crop
sav/crop
sav/cropcrop
sav/crop/rip
sav
sav/cropsav
sav
rip/cropsav
crop/town
sav/crop

wet trop
temp
temp/trop
temp
temp
temp
temp
dry trop
dry trop
dry trop
wet trop
temp
wet trop
wet trop
wet trop
wet trop
dry trop
dry trop
dry trop
dry trop
wet trop
dry trop
wet trop
wet trop

frug/ins
2-4
frug/ins
3-4
frug/ins
2-5
frug/ins
3-4
frug/ins
3-4
frug/ins
3-4
herb/ins
2-4
gran/ins
3-6
gran
5-6
gran
5-7
gran
5-7
gran
3-6
gran
5-6
gran
4-5
gran
4-5
gran /ins
4-7
granparasite
granparasite
gran
3-4
gran
2-5
gran/ins
2-5
gran/ins--omni7
4-6
omni
3-6

Acridotheres ginginianus
Acridotheres fuscus
Acridotheres cristatellus
Pica pica
Urocissa caerulea

E
E
E
I
E

2/1(1)
3/1(2)
51/5(2)
16/5(4)
3/2

town
town
town/riptown/crop
town

n/k
n/k
n/k
crop
n/k

sav/crop
sav/crop
crop
for/sav/cropfor

wet trop
wet trop
temp/trop
temp
temp

omni7
omni
omni
omni
omni

--4-6
4-6
5-9
3-8

*
: figures in parentheses are additional data based on literature.
[References] 1: Yamashina Institute for Ornithology (1979), Kinjo et al. (1987), Kuroda & Komiya (1987), Eguchi & Takeishi (1997), Kinjo (1998),
Tanaka & Takehara (2003), 2: Long (1981), Lever (1987), del Hoyo et al. (1992, 1997), Roberts (1992), Robson (2000), Zheng & Zhang (2002),
3: Tanaka and Takehara (2003), 4: Ornithological Society of Japan (2000), 5: Kinjo et al. (1987), Kinjo (1998), 6: Iozawa (2000), 7: Roberts (1992).

[Abbreviation] Mode of entry: E=escapees; F=feral individuals, I=intentional releases, Habitats/Habitat type: waters= rivers, lakes,
ponds and estuaries; for= indigenous forests, woodlots and bamboo thickets; rip= swamps, reed beds, riparian grasslands andgrasslands; crop= paddy fields, croplands, and orchards; town= parks, town areas, and human habitations; sav= savannas, forest
edges and woodlands; rocky=coastal escarpments, Climate type: temp=temperate; trop=tropic, Diet: omni=omnivore;
herb=herbivore; gran=granivore; frug=frugivore; ins=insectivore; Impact reported: crop=crop damage; hybrid=hybridization;
comp=competition with native species; dis=disease; pred=predation; n/k=not known; ---=no available data.

32

K. EGUCHI and H. E. AMANO

increasing rapidly in indigenous forests (Eguchi &Amano, 2000; Sato, 2000; Kawakami & Yamaguchi,
2004).
Thus, while the Feral Pigeon and Chinese Bamboo
Partridge have reached the integration phase andpenetrated into the community or urban ecosystem,
the Melodious Laughing Thrush and Red-billed
Leiothrix may be in proceeding into the integration
phase. Budgerigars are widely distributed (Fig. 1b),a) Distribution of information

but each population is quite small and is not increasing. Red Avadavats are also widely distributed(Fig. 1c). Although this species sometimes becomes
locally common (Lever, 1987), it does not maintain aself-sustaining population for a long time probably
due to a great year-to-year population fluctuation(Shigeta, 1997). Crested Mynas (Acridotheres cristatellus) are currently limited to a small region (Table 1),
but are increasing rapidly in urban areas in Hyogob) Amandava amandava

No. of information
1-5
6-20
21-

c) Melopsittacus undulatus

Fig. 1d) Leiothrix lutea

Distribution of information and major exotic species. (a) Number of items of information. Data (two items)
from Okinawa Prefecture are not shown in the figure, (b) Red Avadavat, (c) Budgerigar, (d) Red-billed
Leiothrix. In Figs. 1b, 1c and 1d, data for prefectures marked with dark blue and light blue are based on ourinquiries and information from the Ornithological Society of Japan (2000), respectively.

Invasive Birds in Japan

Prefecture (H. Kuroda, pers. comm.). For otherspecies, because sightings are sporadic, they have
hardly been established yet or, if they have, are at avery low density.

4. Impacts of Exotic Birds on Local
Ecosystem and Native Biota
4.1 Influence on agriculture and ecosystem
4.1.1 Crop damage and other economic damage
Economic damage to agriculture and other humanactivities are one of the largest costs of biotic invasion(Pimentel et al., 2000). Because such damage iseasily recognized, many reports of damage caused byintroduced species are available (cf. Long, 1981).
About a half of introduced birds in Japan are agricultural pests overseas where they have been
introduced (Table 1). Particularly, parrots damage
cereals and fruits; waxbills and weavers, cereals; andmynas, cereals and fruits. In the USA, Feral Pigeonsand European Starlings (Sturnus vulgaris) are serious
crop pests, estimated to cause 1.9 billion dollars indamage per year (Pimentel et al., 2000).
In Japan, Feral Pigeons are a serious crop pest,
particularly, on soybeans (Yamashina Institute for
Ornithology, 1979).
The Light-vented Bulbul(Pycnonotus sinensis) causes damage to fruits in
Okinawa Prefecture (Kinjo et al., 1987; Kinjo, 1998).
The high density of the Common Peafowl (Pavocristatus) is a possible cause of crop damages on isletsof the Ryukyu Islands, though no damage to crops has
been claimed because there is no commercial agriculture there (Tanaka & Takehara, 2003). Many
Black-billed Magpies in high density locally build
nests on power poles (Eguchi & Takeishi, 1997),
which causes an increase in electrical accidents.
Rose-ringed Parakeets (Psittacula krameri manillensis)
also are locally common around Tokyo, but noeconomic damage has been reported yet. Because ofvery low densities and restricted ranges, no economic
damage has been reported for other exotic species.
4.1.2 Influence on ecosystem and habitat
Cases of habitat destruction by introduced birds
have been rarely reported. However, if large birds
are introduced into small islands and increase in
number, adverse effects on the environment may bepossible. The expansion of the introduced Common
Peafowl on small islets of the Ryukyu Islands, hasprobably had an adverse effect on both local biota andhabitats. This species is omnivorous. On islets
where peafowl exist in high density, local vegetations
have been modified and some animals including theendemic lizard Eumeces kishinouyei have decreased(Tanaka &Takehara, 2003).
In the Hawaiian Islands, the Common Myna
(Acridotheres tristes) is known as a major seed disperser of the introduced harmful weed, Lantana
camara (Pimentel et al., 2000). Thus, mynas havecaused a change in habitats indirectly by seed33

dispersal of an exotic weed. In Japan, some introduced frugivorous species like mynas andRed-whiskered Bulbuls inhabit town areas andsuburbs where exotic plants are common. If these
exotic birds increase, they may disperse seeds ofweeds and exotic trees to adjacent indigenous habitats.
Of course, one cannot necessarily condemn only
exotic birds for seed dispersal of weeds and exotic
plants; native frugivores like the Brown-eared Bulbul(Hypsipetes amaurotis) do that in the same way.
4.2 Influence on native avifauna
4.2.1 Predation and interference
The Common Myna and Red-vented Bulbul(Pycnonotus cafer) act aggressively sometimes towards native bird species in Mascarenes and Tahiti(Jones, 1996; Thibault et al., 2002; Blanvillain et al.,2003). In Tahiti, native endangered Tahiti Flycatchers (Pomarea nigra) have been decreasing mainly due
to predation by rats. In the process of carrying on arecovery program, Blanvillain et al. (2003) assessed
the impact of Common Mynas and Red-vented Bulbuls on the reproductive success of flycatchers. Nest
failure and death of newly fledged young occurred
often in territories where aggressive interactions byintroduced birds were observed frequently. The factthat a higher number of interactions with Common
Mynas was observed around nests containing eggs andnestlings suggests nest predation by this species(Blanvillain et al., 2003). Common Mynas havebeen introduced into many islands for controlling insect pests (Long, 1981). However, this species isreputed to be a nest thief (Blanvillain et al., 2003).
On the other hand, Red-vented Bulbuls affect flycatchers through aggressive competitive interaction
for food, not direct nest predation (Blanvillain et al.,2003). Aggressive interaction by this species ispresumed to have a possible negative impact also onother monarch (Pomarea) populations in the Polynesian Islands (Thibault et al., 2002). It is likely that
endangered species on a small remote oceanic island
are susceptible to decline of numbers through interference by introduced birds.
If a brood parasite species is introduced, new hostspecies incapable of egg rejection will suffer serious
reproductive losses, e.g., impact of the Brown-headed
Cowbird (Molothrus ater) (Rothstein et al., 1980).
So far, no cases have been reported of predation orbrood parasitism by introduced species in Japan. In
Japan, however, two species of Vidua are known tobreed (Table 1). Vidua species are brood parasites intheir original habitats (Davies, 2000). Although hostspecies have not been observed yet, these species may
influence the breeding of native species.
4.2.2 Disease
A decline in native bird species due to disease
derived from the invasion of introduced birds has been
well documented in the Hawaiian Islands (van Riper
et al., 1986). In the Hawaiian Islands, avian malaria

34

K. EGUCHI and H. E. AMANO

and its vector have accompanied the invasion of introduced birds and reduced the number of native species(van Riper et al., 1986; Dobson & May, 1991). In
the Mascarene Islands, avian pox and parrot diseases
derived from introduced birds have prevailed (Jones,
1996). Species on small and remote islands are
particularly susceptible to new pathogens and parasites brought in by introduced birds.
Furthermore, exotic species living near humanbeings and livestock, such as European Starlings,
House Sparrows (Passer domesticus), Common
Mynas, Feral Pigeons and Rose-ringed Parakeets arereservoirs and vectors of human and livestock diseases,
including parrot fever and ornithosis (Lever, 1987;
Pimentel et al., 2000).
In Japan, no epidemics arising from introduced
exotic birds have prevailed yet. However, there havebeen very few studies conducted on diseases and parasites of Japanese native birds. Several studies haveshown that the prevalence of blood parasites is low in
grassland dwelling warblers and buntings in Japan(Sodhi et al., 1996, 1999; Nagata & Sodhi, 2003).
Introduced birds could act as a large reservoir of various parasites and pass them on to native parasite-freebirds.
Further, the risk of infection with diseases which
also infect humans and livestock like parrot fever orWest Nile fever is high because many caged birds areimported every year (Agency of Environment, 1986;
Nash, 1993).
4.2.3 Hybridization and Introgression
Introduced species can hybridize with closely
related native species. This may cause the extinctionof endangered native species. Sterility in hybrids
results in wasted reproductive efforts, which may
cause a population decline. If hybrids are fertile,
introgressive hybridization results in a loss of original
morphological traits in either species (Rhymer &Simberloff, 1996; Allendorf et al., 2001). Where an
introduced species outnumbers a native species,
original traits of the native species disappear.
In Anatidae, interspecific hybridization occurs
often and, in many cases, hybrids are fertile (Parkin,1996).
In particular, introduced Mallards (A.
plathyrhynchos) or domesticated races have introgressively hybridized with endemic species, causing adecline in the latter (Long, 1981; Hunter, 1996;
Rhymer & Simberloff, 1996). The Hawaiian Duck
(Anas wyvilliana) in Hawaii (Browne et al., 1993),
Florida Mottled Duck (A. f. fulvigula) in south Florida(Rhymer & Simberloff, 1996; Wittenberg & Cock,2001), Australian Black Duck (A. superciliosa rogersi)
in Australia (Long, 1981; Lever, 1987) and Grey Duck(A. superciliosa) in New Zealand (Haddon, 1984;
Gillespie, 1985; Rhymer et al., 1994) have been
decreasing due to introgressive hybridization withintroduced Mallards. The Ruddy Duck (Oxyura
jamaicensis) introduced from North America also
threatens the native White-headed Duck (O. leuco-

cephala) with extinction through hybridization in
Britain (Hughes, 1996).
The case of the Grey Duck is particularly serious.
Gillespie (1985) demonstrated the decrease of pure
Grey Duck populations, based on an analysis of themorphometric variations, due to introgressive hybridization with introduced Mallards in Otago, New
Zealand. In New Zealand, Mallards were introduced
for the first time during the 1860s. Mallards easily
hybridized with native Grey Duck and the hybrids
were fertile. Since the first introduction of Mallards,
the number of pure Grey Ducks has been decreasing.
In 1981-1982, the proportions of pure Grey Ducks,
hybrids and Mallards in samples collected during thehunting seasons were 4.5%, 51.4% and 44.1%, respectively (Gillespie, 1985). By the end of the 1950s, theMallards already outnumbered Grey Ducks andhybrids have increased rapidly since then (Gillespie,1985). Because pure Grey Duck individuals occupy
a very small proportion in the whole Mallard-Grey
Duck collective, hybridization is inevitable. Therefore, local disappearance of pure Grey Ducks is feared.
Rhymer et al. (1994) also demonstrated, based on the
mtDNA analysis, the introgressive hybridization
occurring between these two species throughout thecountry.
Thus, introgressive hybridization between Mallards
and Grey Ducks has expanded drastically to an extent
that some authors have stated that nearly all Grey
Duck populations have become genetic admixtures(Allendorf et al., 2001). In Japan, hybridization
between domestic races of the Mallard and theSpot-billed Duck (A. poecilorhyncha) has also beenreported (Nakamura, 1994), but the impact on native
species is not prominent yet.
In a polytypic species, introduction beyond a rangeof one particular subspecies causes a loss of original
traits of each of the other subspecies adapted to local
environments. Therefore, the release of game birds
beyond the borders of individual locally adapted
populations is harmful.
In Japan, few studies have been conducted on
hybridization among birds in natural habitats. The
Ring-necked Pheasant was introduced onto three mainislands, Honshu, Shikoku and Kyushu, around 1920,
and hybridization with the native Green PheasantPhasianus versicolor occurred (Kuroda & Komiya,
1987; Lever, 1987). Thereafter, introduction wasceased for fear that hybridization would spread inthese islands. On the other hand, introduction wasstarted around 1930 and continued in Hokkaido where
no closely related species lived.
The Green Pheasant comprises four subspecies(Ornithological Society of Japan, 2000). Captive
breeding and release for hunting have been conducted.
However, because release was conducted beyond therange of each subspecies, the uniqueness of eachsubspecies has been lost due to introgressive hybridization among them (Ornithological Society of Japan,

Invasive Birds in Japan

2000). Reintroduction of endangered species should
be carefully planned so as to reduce the likelihood of
hybridization.
4.2.4 Interspecific competition for nest sites
Hole-nesting introduced birds such as the European Starling, House Sparrow, Common Myna andRose-ringed Parakeet can compete with native species
for nesting holes (Long, 1981; Lever, 1987; Feare,
1996; Lindell, 1996, Pimentel et al., 2000). Competition for nesting sites is more severe amonghole-nesters than open-nesters because natural nest
holes are often in short supply (Lindell, 1996).
In North America, European Starlings harass various cavity excavating species like woodpeckers withaggressive interaction and usurp freshly excavatednest cavities. In Mississippi, 52% of nest cavities of
the Red-bellied Woodpecker (Melanerpes carolinus)
were usurped by starlings (Ingold, 1989). In Ohio,
39%, 15% and 14% of nest cavities excavated by
Red-bellied Woodpeckers, Red-headed Woodpeckers(M. erythrocephalus) and Northern Flickers (Colaptes
auratus), were usurped by starlings, respectively(Ingold, 1994). Because starlings and Red-bellied
Woodpeckers start breeding early in highly overlapping periods, the proportion of usurped nest
cavities was highest.
Nest usurpation did not prevent the host species
from breeding at all, but forced them to delay breeding within the season. Usurped Red-bellied Woodpeckers started replacement nests in the vicinity of
their previous nests and eventually fledged young(Ingold, 1994). However, the delayed breeding
caused a decline in fecundity more or less, such as
seasonal declines in clutch size and fledging successdue to a decreased food supply. Therefore, Ingold(1998) concluded that nest usurpation by starlings had
an adverse effect on the breeding success of hostspecies.
Nest usurpation by European Starlings is also
prominent in other species: Gila Woodpeckers,
Melanerpes uropygialis (Kerpez & Smith, 1990) and
Acorn Woodpeckers, M. formicivorus (Troetschler,1976). In all reports so far, the adverse effect of nest
usurpation is unambiguous. However, the demographic data collected in larger areas showed nosignificant long-term declines in native cavity nesting
species. Koenig (2003) compared mean densities ofcavity-nesting species native to North America before
and after the invasion of European Starlings using
Breeding Bird Surveys and Christmas Bird Counts.
Of 27 native species, he could detect a significant
decline in population density after the invasion only insapsucker species (Sphyrapicus spp.). There aresome possible reasons which account for the lack oflong-term declines despite the unambiguous adverse
effect of nest usurpation. Starlings usurping nest
cavities do not always drive host pairs away from their
nesting areas. Therefore, individual woodpecker
pairs whose nests have been usurped excavate a new35

nest in the vicinity of their initial nests and some ofthem eventually fledge young (Ingold, 1989, 1994;
1998).
A delay in breeding may lower their
fecundity. However, the decline in fecundity due to adelay in breeding may be small or nonexistent. For
example, in the Northern Flicker, a significant decline
occurred only in clutch size and not in the number offledglings of the usurped pairs (Ingold, 1998).
Therefore, a large year-to-year fluctuation in reproductive output due to factors other than nest
usurpation may mask the decrease in fecundity due tonest usurpation. This demonstrates the difficulty ofpredicting the effect of invasive species.
In Japan, three species of Acridotheres and theRose-ringed Parakeet have been increasing. Yet nousurpation has been reported of nest cavities of native
species by these species, probably because the distribution ranges are restricted to urban areas so far.
4.2.5 Interspecific competition for foodIn the Mascarene Islands, there are some cases ofinterspecific competition, which have caused a decline
in native species, e.g., the Common Myna and Redwhiskered Bulbul affecting the Mauritius Bulbul(Hypsipetes olivaceus) and Mauritius Cuckoo-shrike(Coracina typica), the Madagascar Fody (Foudia
madagascariensis) affecting the Mauritius Fody (F.
rubra), and the Rose-ringed Parakeet affecting
Mauritius Parakeet (Psittacula eques) (Jones, 1996).
In Hawaiian native forests, generalist species suchas the Japanese White-eye (Zosterops japonicus),
Melodious Laughing Thrush and Red-billed Leiothrix
outnumber native species specializing in a particularfood resource (Mountainspring & Scott, 1985).
Although authors have attributed the decline tocompetition for food, no evidence for the mechanismof competition was shown.
In Japan, the Red-billed Leiothrix has also invaded
broadleaved deciduous forests. This species is dominant over the avifauna of such habitats in Kyushu(Eguchi & Masuda, 1994; Eguchi & Amano, 2000).
Among the avifauna of broadleaved deciduous forests,
major species occupying a similar niche to this species
are the Japanese Bush-warbler (Cettia diphone), GreatTit (Parus major), Varied Tit (P. varius), Coal Tit (P.
ater), Willow Tit (P. montanus) and Long-tailed Tit(Aegithalos caudatus). Although there is no evidence to confirm the presence of interspecific competition between introduced and native species for foodresources or nest sites, predation on Japanese Bushwarblers nesting in the same nesting habitat as the
Red-billed Leiothrix has been found to be higher thanin other regions where the latter species has not yet
invaded (Amano & Eguchi, 2002a, 2002b). Even if
there is no direct competition, a high density of the
Red-billed Leiothrix may cause a decline in reproductive success in sympatric species through “apparent
competition” (Martin & Martin, 2001) mediated byattraction of predators.
Kawakami and Higuchi (2003) could not detect

36

K. EGUCHI and H. E. AMANO

any negative interspecific relationships between theendemic Apalopteron familiare and introduced
Japanese White-eye in spite of similar food habits in
the Bonin Islands.
Thus, it is even more difficult to demonstrate theexistence of direct interspecific competition for foodthan for nest-sites. So far, in some cases, native bird
species are presumed to have decreased in number due
to interspecific competition with introduced species.
However, no demonstration with quantitative evidence
has been done in either case. Introduced species arecommon in disturbed habitats like croplands and town
areas where native species have already disappeared(Green, 1984; Blair, 1996; Case, 1996; this study).
Therefore, they may compete with other introduced
species (Moulton & Pimm, 1986). In indigenous
forests, introduced species interact with native species.
Because the habitat structure is complicated and foodresources are diverse there, we might be unable to finda negative interspecific relationship between native
species and introduced species successfully invading
such a habitat. If so, interspecific competition forfood is usually not severe between closely related
species or between species belonging to the sameguild, contrary to the competition for nest holes. In
most instances where an introduced species has
exterminated a native species, the species interactions
were between trophic levels (Davis, 2003). Probably,
a top-down effect caused by introduced species causes
more serious results for native species. For example,
the Common Peafowl in the Ryukyu Islands modified
the vegetation and caused a decrease in some preyanimals, which may have caused a decline insympatric birds such as White-eyes (Tanaka &Takehara, 2003).

5. Invasiveness of Introduced Birds in Japan
In total, 79 exotic species were sighted out of
captivity in our inquiry investigation. Forty-three ofthem were likely to breed. However, there have beenfew species establishing self-sustaining populations so
far. Most exotic species arrived not so long ago. It
took only a couple of decades for the Red-billed
Leiothrix to spread throughout Kyushu (Amano &Eguchi, 2002a). Hence, it is likely that the numberof species establishing a self-sustaining population
will increase during the next decade.
There is no doubt that the success of introduction
is determined primarily by the effort put into introducing a species, i.e., the size and number of releases(Veltman et al., 1996; Duncan, 1997; Green, 1997).
However, if efforts to introduce them are controlled,there are significant relationships between some ecological traits of both the introduced species and therecipient community and the success of introduction;higher success rates are seen in species which are
more sedentary (Veltman et al., 1996; Cassey, 2002),
have a larger body mass (Green, 1997; Duncan et al.,1999), small clutch size (Green, 1997), higher level ofbehavioral flexibility (larger relative brain size, higher
frequency of foraging innovation and higher habitat
generalism; Sol & Lefevre, 2000; Sol et al., 2002;
Cassey, 2002) and originate from latitudes closer tothose of the areas into which they are being introduced(Blackburn & Duncan, 2001). Species with smaller
clutches may have a lower reproductive rate but survive longer, and it is not clear whether reproduction orsurvival is more important (Green, 1997). Smallerdifferences in latitude may mean that the species isbeing introduced into areas with the same climate as
the original habitats and can establish itself successfully (Blackburn & Duncan, 2001). However, ecological traits are not always a reliable predictor in assessing invasiveness of each species (Willenberg &Cocks, 2001). Outcomes of introduction are often
different between related species. Simberloff and
Boecklen (1991) stated that the most reliable predictor
is whether or not the species was successful at establishing itself in other areas.
Systematic and large-scale introductions, such as
those by acclimatization societies, is no longer done.
A typical pattern of establishment is that escaped
caged birds gather and breed by chance. However,
ecological traits of both the introduced species and therecipient community and abiotic factors of habitat
may determine whether their number increases or not.
Most exotic birds breeding in Japan are from the tropics (Table 1). In general, birds in the tropics have along breeding season and lay several clutches within asingle season. Indeed, the Red Avadavat in Japan
breeds out of the ordinary season when most native
birds breed (Shigeta, 1997). The Red-billed Leiothrix
has a longer breeding season than native species(Eguchi & Amano, unpublished data). If global
warming progresses, it may become more likely thatexotic species from the tropics will establish themselves and increase in Japan.
The hole-nesting Rose-ringed Parakeet and mynas
currently inhabit town areas. However, they may
begin to compete with native hole-nesting species ifthey increase and extend their ranges towards rural
areas.
Many species closely related or identical to
Japanese races have been imported as pets mainly
from the Eurasian continent (Nash, 1993). It isfeared that these birds may hybridize with Japaneseraces and that the original traits of the Japanese races
will disappear through introgressive hybridization.
Currently, there are only a few exotic species withhigh density, and serious adverse effects are prominent
for only several species: the Feral Pigeon, Lightvented Bulbul and Common Peafowl. However,closely related or identical species have increased inafter being introduced into areas other than Japan andhave damaged native biota and crops there (cf. Long,
1981; Lever, 1987). Even if only a few pairs breed
currently, the risk that these species will increase andInvasive Birds in Japan

become a threat to native biota and habitats in the near
future is high. Most exotic birds in Japan areomnivores, granivores, or frugivorous insectivores.
These species may easily become crop pests if theyoccur at high densities.
Finally, the effects of introduced birds on ecosystems, biota and human society are not obvious.
Most introduced birds inhabit disturbed habitats (e.g.,
Blair, 1996; Case, 1996). Therefore, introduced
birds replace native species that have disappeared due
to human disturbance, and hence play a role inmaintaining ecosystems and mitigating environments
for human beings (Case, 1996). For example, introduced game birds occupy a similar niche held by
now-extinct or rare native species and facilitate seeddispersal and germination of native plants in disturbed
habitats in Hawaii (Cole et al., 1995). Therefore,
people believe that because introduced birds areharmless compared to other animals, they should beallowed to exist in disturbed habitats until they start toexhibit negative impacts on the ecosystem andhumans. However, proliferation of introduced species in local regions means a loss of unique regional
avifauna and facilitates homogenization of fauna on a
global level. Rather, we should try to regenerate disturbed habitats and create room for native birds toreturn there.

Acknowledgements
This study is partly supported by the Pro Natura
Fund from the Nature Conservation Society of Japan,
a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science andTechnology of Japan (No. 08454252), and a Grant for
the Global Environment Research Program (No. F-3(2001-2003)) by the Ministry of the Environment ofJapan. The following organizations helped us in theinquiry into introduced birds: the OrnithologicalSociety of Japan, Japanese Society for Preservation ofBirds, Wild Bird Society of Japan (WBSJ), Sapporo
Branch of the WBSJ, Kitakyushu Branch of the WBSJ,
mailing list “jeconet”, and mailing list “banding”.
We thank all the people who have provided invaluable
information about exotic birds in Japan, especially Dr.
M. Izawa, Mr. Y. Nakamura, Mr. M. Kamogawa, Mr.
S. Nakamura and Mr. H. Kuroda.

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English names and Scientific names of invasive birds in Japan.

Order
Anseriformes

FamilyAnatidaeCiconiiformes
Galliformes

Ciconiidae
Phasianidae

Columbiformes
Psittiformes

Columbidae
Psittacidae

Passeriformes

Pycnonotidae
Timaliidae

Emberizidae
Estrildidae

Ploceidae

Sturnidae

CorvidaeEnglish name
Mute Swan
American Wood Duck
Muscovy Duck
Painted Stork
Chinese Bamboo Partridge
Ring-necked PheasantNorthern Bobwhite
Common Peafowl
Feral Pigeon
Budgerigar
Monk Parakeet
Alexandrine Parakeet
Rose-ringed Parakeet
Moustached Parakeet
Red-whiskered Bulbul
Linght-vented Bulbul
Spectacled Laughing Thrush
White-browed Laughing Thrush
Melodious Laughing Thrush
Red-billed Leiothrix
Red-crested CardinalBlack-rumped Waxbill
Orange-cheeked Waxbill
Nutmeg Mannikin
Tricolored Mannikin
Chestnut Mannikin
White-rumped Munia
White-headed Mannikin
Java Sparrow
Red Avadavat
Paradise Whydah
Pin-tailed Whydah
Yellow-crowned Bishop
Red Bishop
Streaked Weaver
Lesser Masked Weaver
Asian Pied Starling
Common Myna
Bank Myna
Indian Jungle Myna
Crested Myna
Black-billed Magpie
Formosan Blue Magpie

Scientific name
Cygnus olor
Aix sponsa
Cairina moschata
Mycteria leucocephalaBambusicola thoracica
Phasianus colchicus karpowi
Colinus virginianus
Pavo cristatus
Columba livia
Melopsittacus undulatus
Myiopsitta monachus
Psittacula eupatria
Psittacula krameri manillensis
Psittacula alexandri
Pycnonotus jocosus
Pycnonotus sinensis
Garrulax perspicillatus
Garrulax sannio
Garrulax canorus
Leiothrix lutea
Paroaria coronata
Estrilda troglodytes
Estrilda melpoda
Lonchura punctulata
Lonchura malacca
Lonchura atricapilla
Lonchura striata
Lonchura maja
Padda oryzivora
Amandava amandava
Vidua paradisaea
Vidua macroura
Euplectes afer
Euplectes orix
Ploceus manyar
Ploceus intermedius
Sturnus contra
Acridotheres tristis
Acridotheres ginginianus
Acridotheres fuscus
Acridotheres cristatellus
Pica pica
Urocissa caerulea(Received 8 January 2004, Accepted 3 February 2004)