The Condor 93:383-389
Q The CooperOmithobgical Swiety 1991

GEOGRAPHICAL VARIATION OF THE PLUMAGE
POLYMORPHISM IN THE EASTERN REEF HERON(EGRETTA SACRA)’SINGI ITOH*

Laboratoryof Animal Sociology,
Facultyof Science, Osaka City University,
Sugimoto, Sumiyoshi-ku, Osaka 558, Japan
Abstract. I investigated the distribution of white and dark morphs of the Eastern Reef
Heron (Egretta sacra) on severalcoastsand islandsin central to southwesternJapan, eastern
Australia, and French Polynesia. Both morphs preferred white beachesto dark beaches.
Every local population contained some dark herons, however, the rangeof the white morph
was restricted. The white morph population increasedin proportion to the distance fromthe equatorin Japan. It was plentiful in the Great Barrier Reef in Australia and was common
in Polynesianislands. The distribution of the white morph coincides with the presenceof
coral reefs, from which white beachesare derived. Only the dark morph was seen outside
this range.None of severalhypothesesproposedso far explainsthe observedpolymorphism
and its geographicvariation.
Key words: Color uolvmoruhism: Dlumaze color; geographic variation; coral reefs; Eastern
Reef.Heron; Egretta sac&__‘-

INTRODUCTION

techniques while dark herons forage in a more
active manner. This hypothesis was questioned
subject to natural selection, the coexistence of by Recher (1972) who did not find any differencein foraging behavior between the two morphs ofwhite and dark conspecifics presents an interesting paradox. Such “white-or-dark” type of the Eastern Reef Heron (Recher and Recher1972). Instead, Recher and Recher suggestedthat
plumage polymorphism is known in several avian speciessuch as the Parasitic JaegerStercora- physiological adaptation to hot coastal habitatsrius parasiticus and relatively common in the may have played an important role; white birds
Ardeidae (egretsand herons). Four to six species, may be favored in hot areas becauseof less abdepending on the taxonomy, are known to be sorption of sunlight. Walsberg et al. (1978) however, showed that white plumage may actuallypolymorphic within some local populations, independent of age and sex (Mock 1980, Hancock absorb more heat than dark plumage. Balanced
and Kushlan 1984): the Great Blue Heron Ardea selection forcesbetween risk of predation (Caldherodias, the Reddish Egret Egretta rufescens, well 1986) and the advantage of “hunting camthe Little Egret E. garzetta, the Madagascar Reef ouflage” (Craik 1944) for white birds may contribute to maintain polymorphisms in Ardeidae.
Heron E. dimorpha, the Western Reef Heron E.
It is obvious that more quantitative data on thegularis, and the Eastern Reef Heron E. sacra. All
except the Great Blue Heron live almost exclu- morph ratios and other aspectsare required totest such hypotheses(see Mock 1980).
sively in coastal habitats.
The Eastern Reef Heron is a western Pacific
There are several hypothesesregarding plumage polymorphism in Ardeidae. Murton (1971) coastal heron that exhibits two distinctive color
proposed that white herons, which he thought morphs: white and dark. On the basisof censuses
lessconspicuousprey than dark ones, use “stand in several Polynesian islands, Holyoak (1973)
and wait” or “wade slowly and surprise” foraging found a close correlation between morph ratios
and beach colors, the white morph being moreprevalent on islands with predominantly white
I Received3 1 August 1990. Final acceptance14 Jan- beaches.
uary 1991.
I investigated the distribution of the polyz Singi Itoh died in April 1990. This paper was submorphic
Eastern Reef Herons and their habitats
mitted by his advisor, S. Yamagishi, with minimum
modifications.All correspondenceshouldbe addressed at more localities acrossa wider range than did
to Dr. Yamagishi at the above address.
Holyoak, including southern Japan and several
Given that conspicuousnessand crypticity are

[3831

384

SING1 ITOHTABLE 1. Study areas and the numbers of Eastern Reef Herons observed in Japan (A), Australia (B) and
French Polynesia (C).
(4 Japan
LocationNO.

Studyarea

N

E

Jl.
52.

MikunichB
Kii Peninsula

35’15, 136Q7
330301,135”45’

53.
J4.
J5.
J6.
57.
:;:
JlO.

South Kyushul
Kuchierabu Is.*
Tanega Is.
Yaku Is.
Tokara Arch.)
Amami Gshima
Yoron Is.
Okinawa Is.

3 l”OO’, 130”40’
30”28’, 13o”lO’
30”44’, 131”oO’
30”25’, 130”35’
29”10’, 129”13’
28”22’, 129”30’
27”10’, 127”27’
26”12’, 127”40’

Jll.
512.
513.
514.
J15.
516.
517.
518.
J19.

Aguni Is.
Aka Is.
Miyako Is.
Minna Is.
Tarama Is.
Ishigaki Is.
Kohama Is.
Iriomote Is.
Yonaguni Is.

26”35’, 127”14’
26”11’, 127”17’
24”48’, 125”17’
24”45’, 124”42’
24”39’, 124”42’24”2l’, 124Q8
24”21’, 123”59’
24”10’, 123”45’
24”28’, 123”OO’

Dates

28-29 Mar. 1987
9-13 May 1988
13-14June 1988
22 Feb. 1988
21 Feb. 1988
22-23 Feb. 1988
20 Feb. 1988
18-20 Feb. 1988
13-20June 1987
16-17 Feb. 1988
6-8 Feb. 1988
2-5June 1988
9 Sep. 1988
11 Sep. 1988
15-16 Feb. 1988
7 Sep. 1988
6 Sep. 1988
9-11 Feb. 1988
4 Sep. 1988
12-13 Feb. 1988
10 Feb. 1988

HOUIS

Mqhs
Dark
White

12
64

12
16

4
4
27

-

-

3:
74
12
55

1
19
2
2
35
7
31

0
0
0
2
20
5:

0
1
19
2
4
55
12
86

8
10
13
6
8
22
7
18
3

8
4
15
3
21
14
2
14
1

3
2
10
19
25
32
4
26
5

11
6
25
22
46
46
6
40
6

HOUS

Morphs
Dark
White

0
0

Total no.
of herons

1216(B) Australia
Location
NO.

Al.
A2.
A3.
A4.
A5.

Study area

Green Is.
Fitzroy Is.
Wilson Is.
Heron Is.
Lord Howe Is.

S

E

16”43’, 146”OO’
16”54’, 146QO’23”20’, 151054’
23”27’, 151”55’
3 1”28’, 159”09’Dates

4-5 Feb. 1988
15 Dec. 1989
20 Dec. 1988
13-15 Nov. 1988
23-25 Nov. 1988

10
5
212
7-

12
2
46

84
2
137
-

Total no.
of herons

96
4
0
183
0

(C) FrenchPolynesia
LocationNO.

Studyarea

Pl.
P2.
P3.

Raiatea
Borabora
Tuamotu Arch.

S

W

16”50’, 151”30’
16”30’, 151045’
15QO’, 147”40’

Dates

HOIUS

2 Dec. 1989
3 Dec. 1989
23 Nov.-9 Dec. 1989

2
4
10Morphs
Dark
White

9
9

2
16

Total no.
of herons0
1125’ Consistsof two localities,about 30 km apart.

2IncludesIO and Take Islands.
’ Consistsof nine small islands.

other Pacific islands in the Southern Hemisphere. My data indicate great geographic variations in morph ratios and that the distributionof the white morph coincides with that of coral
reefs. I report these results with discussion on
the possible selection force contributing to maintain the color polymorphism.
STUDY

AREA ANDMETHODS

Censuses were carried out by one to five person(s)for a total of 438 hr during 6 1 days from March
1987 through December 1989 in 19 localities inJapan, five in Australia and three in French Polynesia (Table 1). Two census methods were employed. 1. The observer(s) walked or drove at a
speed of about 1 km/hr along the coasts (or all
the areas on islands that were reasonably small),and recorded the numbers of the color morphs(white or dark) and their habitats (also white ordark). 2. When it was not possible to land orwalk along a coast, we counted Reef Herons thatwere flushed from the coasts as our boat passed
50-l 50 m away.
Habitat color was divided into two categories.

PLUMAGE POLYMORPHISM

.

EAST
CHINA

IN REEF HERONS

385

-32
JS

SEA

24

26

26

30

32

34

36

LATITUDE

FIGURE 1. The correlation betweenlatitude and the
proportion of white Eastern Reef Herons in Japan.
Kendall’s rank correlation test: n = 16, S = 7 1, P =0.001 when all data are used: n = 13, S = 32, P =0.05 1 when northern three localities, where all herons
were dark, are excluded.

“Dark beaches” consisted of dark rocks and/or
sands. “White beaches” consistedof limestones
or white sands (sometimes with scattering darkrocks).Habitat color was recordedon maps when
the bird censuseswere made; beach lengths were
measured from the maps. In any of the northern
localities in Japan (51-56 in Table l), no whitebeacheswere found, while no dark beacheswere
observed in Australia and Polynesia. However,there was one special circumstance on Heron
Island. Although dark fringed substrates surrounded this island, Reef Herons did not feedon theseareas.This apparently was due to heavy
tourist use. In this paper, therefore, Heron Island
is classifiedas a white beach on which the herons
fed.
At three localities in Japan, two in Australia,
and one in Polynesia, fewer than four herons(Table 1) were observed, so these were excluded
from further analyses.Most correlation analyses
were made for data from the Nansei Islands, Japan (the Tokara Archipelago, Amami Islands,
and Okinawa Islands: 57-J 19 in Table 1) because
these islands provided variations in both beachtypes and bird color.
RESULTS
DISTRIBUTION

OF THE TWO MORPHS

In all, I counted 415 Eastern Reef Herons inJapan, 283 in Australia, and 36 in French Polynesia (Table I), with 208 (50.1%) 221 (78.1%)

and 18 (50%) white morphs.

FIGURE 2. The distribution of color morphs of E.
sacra in Japan with isothermal lines for the coldest
monthly sea surface temperature (20°C Feb. 19561985). The numbers coincide with those in Table 1.
The broken line is Watase Line (see text). The circles
indicate the proportion of white and dark morphs ineach locality as blank and shaded parts, respectively.
See Table 1 for sample sizes.

The proportion of the white morph decreased
as latitude increased in Japan (Fig. 1). The correlation was weak when northern localities with
only dark herons were excluded.
The sample size in the Southern Hemisphere
was too small to make a similar correlation analysis. The proportion of the white morph was highin both Green (Al), Fitzroy (A2) and Heron (A4)
Islands in the Great Barrier Reef, Australia:
87.5%, 50.0% and 74.9%, respectively. Eastern
Reef Herons were plentiful there; the total number of herons (283) found in these small islands(0.3 and 0.17 km2) is more than 77% of that inmany of the Nansei Islands, where intensive censuseswere made. In Polynesia, we found 11 Reef
Herons with two (18.1%) white morphs in Borabora and 25 herons with 16 (64.0%) white onesin Rangiroa. All beaches we examined in Australia and Polynesia were white.
Every local population contained some dark
Eastern Reef Herons, whereas the range of thewhite morph was restricted. Takara Island in the
Tokara Archipelago (57) was the northern limitof white Eastern Reef Herons (Fig. 2). All 50
birds were the dark morph in the north of theWatase Line, a biogeographicboundary that runs
through the Tokara Straitsat the latitude of 3O”N,
while 208 (56.9%) birds out of 365 herons were386

SING1 ITOH
100

1

0’

I

50
% OF

WHITE

100
BEACHES

Reef Herons were observed on white beaches
than on dark ones (x2 = 23.222, P < 0.001).
Eastern Reef Herons show no preference forbeach habitats that match their plumage. The
proportion of white birds did not differ between
white and dark beacheswithin each island (Table3). White morphs were seen on dark beachesin
10 of 11 islands and, conversely, dark morphs
were seen on white beachesin all 12 islands. The
same is true even if the data sourceis restricted
to the 10 islands where there are both types ofbeaches. On the basis of pooled data, 56.7% ofEastern Reef Herons seen on white beachesand
57.5% of ones on dark beaches were the whitemorph (x2 = 0.019, P = 0.890).

FIGURE 3. The correlationbetweenthe proportionofwhitebeaches
andthatof whiteEasternReefHerons. DISCUSSION
Kendall’s rank correlationtest:12= 16, S = 42, P =Several birds exhibit a “white-or-dark” type of
0.059 when all data are used;n = 10, S = 15, P =0.180 when localitiesthat do not containeitherdark polymorphism, independent of sex and age,
within populations as doesthe Eastern Reef Herbeachesor white onesare excluded.
on. Genetical studieshave shown that thesepolymorphisms are usually controlled by two alleles,
the white morph in 13 islands in the south of the with various extent of dominance, at a single
locus (the Antarctic Giant Petrel Mucronectes
line (Fig. 2).
The proportion of white herons was not sig- gigunteus:Shaughnessy1970, the Parasitic Jaeger: O’Donald and Davis 1959, the LesserSnow
nificantly correlated with that of white coastsin
Japan (Fig. 3), whether including or excluding Goose Chen caerulescenscaerulesens:Cooke andlocalities that lacked either dark beachesor white Mirsky 1972, the Little ShagPhalacrocorax melanoleucos brevirostris: Dowding and Taylor
beaches.
1987). Using family analysis, Naik and ParaHABITAT CHOICE BY THE TWO MORPHS
sharya (1983) suggestedthat the polymorphism
Table 2 summarizes the numbers of Reef Herons
in the Western Reef Heron is also determined
observed in the Nansei Islands with respect toby a single pair of alleles with the gene for dark
beach colors. In three islands (Amami-Gshima
being incompletely dominant over the gene for[J8], Okinawa [Jlo] and Miyako [J131)out of thewhite. It seems likely, therefore, that the ob10 islands where both white and dark beaches served phenotypic variation in plumage color inexist, herons of both colors were significantly
the Eastern Reef Heron is based on genotypic
more abundant on white beaches than on darkvariation similar to that of other white-or-darkones. The same tendency was found in four ofbirds.
the other seven islands, although the difference
What evolutionary mechanism contributes towas not statistically significant. The frequencies maintaining the phenotypic polymorphism withof the two beach types were almost identical on
in local populations of Reef Herons and what are
one island. More herons were observed on dark
the selective advantages, if any, of being either
beachesthan white ones in Aguni (J 11) and Kowhite or dark? Becauseindividuals of eachmorphhama (J 17) Islands, though thesedifferenceswere are quite uniform, the statussignaling hypothesis
not significant. This may be becausemost of the(Rohwer 1977) and the individual recognition
white beachesin Aguni Island were too narrow
hypothesis (Shields 1977) are not applicable towith steepcliffs nearby for herons to forage, andthe Eastern Reef Heron.
Kohama Island was censused when the whiteThis study revealed a consistent geographic
beacheslooked like mud flats that are not prefvariation in morph ratios. The boundary beerable habitats for Eastern Reef Herons. When
tween the region where both color morphs occur
all records for the 13 islands are pooled, more
and the region where only the dark morph occurs

PLUMAGE POLYMORPHISM IN REEF HERONS

387

TABLE 2. The distributions1of beachcolorand the two colormorphsof E. sacra in the NanseiIslands.

Locality’

:i

J::
Jll
512
513
514
J15
516
517
J18
J19

Length
Oan)

67.0
16.0
184.7
7.0
6.1
41.5
0.7
10.5
5.5
4.7
12.4
356.1

Dark beaches
MorphsTotal no.
Dark
White
of herons

6
3
14
7
2
?l
12
0
1
1
48

z

7
43
10
3

!:
15
:

2
27
1
4
6
113

5
65

Length
(km)

s

4
29
3
1

-

Birds/
km

0.12
0.44
0.23
1.43
0.49

*
*

0.05
2.86
2.57
0.18
0.85
0.48

*

0.31

*

4.6
126.6

White beaches
MorphsTotal no.
of herons
Dark
White

Birds/
km

6;‘:
2:o
6.2

2
29
4
17
1
2

2
18
1
26
0
1

4
47
5
43
:

0.87
0.37
1.28
0.69
0.50
0.48

27.0
5.7
6.1
75.6
9.0
50.6
319.3

13
3
9
14
1
14
109

17
10
10
31
1
26
143

23
20
19
45
2
40
252

0.85
3.51
3.11
0.60
0.22
0.19
0.66

’ It was testedwhetherthe distribution differedfrom expectedvalue on the basisof the beachlength,usingthe binomial test or x2-test.Asterisks= P < 0.05; the others= N.S.
1 The numberscoincidewith thosein Table 1.

is in the Tokara Strait (the Watase Line) in Japan(Fig. 2). Eastern Reef Herons of both types live
south of the Watase Line, and only dark herons
live north of it. Kiyosu (1978) reported that all
81 Reef Herons found in Honsyu, north up to
Aomori (40”49’N, 140”49’E), were the darkmorph. The Watase Line also correspondswith
the northern limit of coral reefs(Takahashi 1988).
That is, coral reefs develop well in the south of
the Nansei Islands, decrease drastically from
Amami 6shima to Takara Island, and vanish in
the north of the Tokara Strait (Takahashi 1988).
The same pattern has been found in theSouthern Hemisphere. MacDonald (1973) describes that “the white morph gets increasingly
common towards north and rare towards south.”
Both morphs have been recorded in the Great
Barrier Reef and Caledonia (Mayr and Amadon
194 1, this study), while only dark morphs havebeen seen farther in the south, between Sydney(33”52’S, 15 l”12’E) and the Victorian-Border (H.
F. Recher, pers. comm.), Tasmania and New
Zealand (Falla et al. 1970). In short, the whitemorph of Eastern Reef Herons apparently increasesin proportion as the distance from theequator increasestowards both north and south,
although the documentation for the southernhemisphere remains a bit sketchy. This patterncoincideswith the distribution of coral reefs,fromwhich white beachesare derived.
Non-adaptive polymorphism can be an out-

come of recent merging of two formerly allopatric populations as in the Lesser Snow Goose(Cooke et al. 1988) or maintained by immigration of individuals with a phenotypic preference
for marginal habitats where another competitive
phenotype has a selective advantage. In the Ardeidae, all the three coastal species are polymorphic (the Western Reef Heron, the Eastern
Reef Heron, and the Reddish Heron). White
morphs are generally predominant at lower latitudes, as observed in this study, except in the
Red Sea (seePalmer 1962, Mock 1980, Hancock
and Kushlan 1984 for brief reviews). Such con-TABLE 3. Beachcolor preferenceof the two color
morphsof E. sacra in theNanseiIslands.Islandswhere
only dark or white beachesexistare excluded.

Locality’

J8
J:;:
Jll
512
513
514
J15
516
517

Percentof white birds
White beaches
Dark beaches

25.0
57.1
61.4
30.0
33.3
0.0
55.6
100.0
100.0
75.0

I The numberscoincidewith thosein Table I

38.3
20.0
60.4
0.0
33.3
43.5
52.6
85.0
68.9
50.0

388

SING1 ITOH

sistentgeographicvariation seemsto favor adaptive explanations to non-adaptive ones.
Murton (197 1) presented a hypothesis thatwhite and dark morphs in herons are maintained
becausethe white forms are less visible to preyand have an advantage in situations demanding
stealth or surprise while the dark morphs havean advantage in scaring or flushing prey out of
hiding or into movement. Recher (1972), however, argued that Murton’s hypothesisis unlikely
since there is no difference in foraging behavior
between the two morphs of the Eastern Reef Heron (Recher and Recher 1972). In addition, interspecific comparison of foraging behavior
among North American herons doesnot provide
any correlation between foraging techniquesand
body color (Kushlan 1978).
Recher (1972) suggestedthat the dimorphisms
may have a physiological rather than an ecological reason; white herons hunting on hot coasts
may be better able to regulate their body temperature than dark ones. This hypothesis coincides with the result of the present study, thenegative correlation between the latitude and the
proportion of the white morph (but seeHolyoak
1973 for an exception). Ellis’ (1980) laboratory
experiment indicates that metabolism and solar
radiation may combine to amplify heat stressin
hot climates in dark herons.
Matching with different habitats may seemstraightforward as an explanation of polymorphism. Holyoak (1973) examined the relationship between beach color and the morph ratios
in Eastern Reef Herons in French Polynesia.
There were highly significant differencesbetween
the morph ratios of islands with beachesof different colors,thoseislands with light coral beaches had far more white herons. The white morph
was absent from coastswith no coral, as in New
Zealand and southern Australia, and present
wherever white coral beaches are within easyreach. These findings were not confirmed by thisstudy because the correlation between habitat
color and morph ratios was not significant. The
current study, furthermore, revealed that individual herons did not choose habitats of theirown color. Therefore, the observed geographic
variation in morph ratios requires explanations
at the population level such as frequency-dependent selection or immigration.
Given that habitat matching plays an important role in maintaining the polymorphism inherons,the advantageof habitat matching should

also be explained. Caldwell’s (1986) experimentshowing that white heron models attracted more
predators than blue models suggeststhat morphs
matching their backgrounds are at less risk frompredation. Alternatively, it has been shown thatthe white underparts of herons and seabirds facilitate foraging becauseof their inconspicuousnessto aquatic prey against the bright sky (Mock
1980, Gotmark et al. 1986, Gotmark 1987). Thebalanced selection of these two selection pressuresmay contribute to maintain two morphs inherons.
None of the above fully explains polymorphism in Ardeidae. Becauseall the three typicallypolymorphic herons are marine birds, changes
in activity patterns with tides may be important.
It thus is interesting that Eastern Reef Herons
are active at night as well as in the daytime (Itoh,pers. obs.), considering that nocturnal speciesin
Ardeidae have dark plumage. To test any of thesehypotheses,further intensive studiesare needed.
ACKNOWLEDGMENTS
I would like to thank Satoshi Yamagishi for his enthusiasticguidanceand encouragementthroughoutthis
study. I am grateful to Masahiro Fujioka for criticalreading and English translation of the original manuscript, to Jiro Kikkawa, Eiichiro Urano and Yoshito
Ohsako for helpful suggestions,and to Masashi Yamaguchi and Motoharu Koba for valuable suggestion
on geographicaland morphometrical studies of coral
reefs.Thanks are alsodue to Ryozo Kakizawa, Tamiko
Itoh for their help with analysesof specimens,to James
Kushlan, Harry Recher, Hiroyuki Morioka, Miyako
Tsurumi for their help with obtaining literatures, andFusako Itoh, Yumiko Itoh, Chiyo Takada, Wakako
Nagata, Naomichi Sato, Yoshiya Ueno, ShinsukeNakai, Liz Edwards for their providing assistancein thefield work.

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COOKE,F., D. T. PARKIN,ANDR. F. ROCKWELL.1988.
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DO\KDING,J. E., AND M. J. TAYLCIR. 1987. Genetics
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5 l-57.
ELLIS,H. I. 1980. Metabolism and solar radiation indark and white herons in hot climates. Physiol.
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IN REEF HERONS

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