Zootaxa 2326: 24–38 (2009)www.mapress.com / zootaxa/

ISSN 1175-5326 (print edition)

ArticleCopyright © 2009 · Magnolia PressZOOTAXAISSN 1175-5334 (online edition)

Taxonomy and distribution of the Pygmy Eagle Aquila (Hieraaetus) weiskei(Accipitriformes: Accipitridae)
JAN O. GJERSHAUG¹, HEATHER R.L. LERNER² & OLA H. DISERUD¹¹Norwegian Institute for Nature Research, Tungasletta 2, 7485 Trondheim, Norway.
E-mail: jan.o.gjershaug@nina.no; ola.diserud@nina.no²Smithsonian Institution, Center for Conservation and Evolutionary Genetics, National Zoological Park, Genetics Lab, 3001
Connecticut Avenue, NW, Washington, D.C. 20008, USA. E-mail: lernerh@si.edu

Abstract
We re-evaluated the taxonomic status of the New Guinean subspecies weiskei of the Little Eagle Aquila morphnoides andconfirmed that it differs considerably from the Australian A. morphnoides in multiple morphological and plumage
characters. Genetic differentiation in published mitochondrial cyt-b and ND2 sequences between the nominate
subspecies and weiskei is equivalent to that found between other accipitrid taxa at the species level, as are pairwise
divergences between each Little Eagle taxon and their sister species, the Booted Eagle A. pennata. The available
morphological and genetic evidence thus strongly and unambiguously supports the specific distinctness of A. weiskei.
We propose the English name Pygmy Eagle, as this is the world’s smallest known eagle. We show that the distribution of
A. weiskei includes the western part of New Guinea (Vogelkop) and the Moluccas (Halmahera, Ternate and Seram),based on our new observations.
Key words: morphology, genetics, pennata, morphnoides, Little Eagle, New Guinea, Moluccas

Introduction
The Little Eagle Aquila morphnoides was described by Gould (1841) from Australia. Reichenow (1900)
described a similar taxon from New Guinea as Eutolmaetus weiskei. The latter taxon was later regarded as asubspecies of Hieraaetus morphnoides (Peters 1931; Stresemann & Amadon 1979). Dementiev et al. (1966)
treated Hieraaetus as a synonym of Aquila. The genera Aquila and Hieraaetus (sensu Stresemann & Amadon1979) are now known to be paraphyletic, and most of the species of the latter genus may be included in Aquila(Wink & Sauer-Gürth 2004; Helbig et al. 2005; Lerner & Mindell 2005; Gjershaug 2006; Haring et al. 2007),
which has priority (ICZN 1999). The Taxonomic Sub-Committee of the BOU Records Committee (Sangster
et al. 2005) treats the Booted Eagle Hieraaetus pennatus as Aquila pennata. The name Hieraaetus can be used
as a subgenus name for the ‘booted eagle’ clade (Parry 2001) that includes A. morphnoides, Pygmy Eagle A.
weiskei, A. pennata, Ayres’s Hawk-Eagle A. ayresii Gurney, 1851 and Wahlberg’s Eagle A. wahlbergi
Sundevall, 1851 (Helbig et al. 2005). Alternatively, Aquila and the revised Hieraaetus (sensu Helbig et al.
2005) may be treated as separate genera, and there is some behavioural, vocal and morphological support forsuch a distinction (Debus et al. 2007), but here we follow Sangster et al. (2005).
Brown and Amadon (1968) mentioned that the New Guinea race of Hieraaetus morphnoides issometimes regarded as a separate species, but did not provide a reference. Amadon and Bull (1988) indicatedthat the diminutive form weiskei of New Guinea may be a separate species. Parry (2001) suggested that A.
weiskei should be treated as a valid biological species based on character differences (in both juvenile andadult plumages) and biometric differences between this taxon and its sister A. morphnoides. He (Parry 2001)
maintained that these differences are greater than the differences between A. morphnoides and A. pennata,24

Accepted by P. Rasmussen: 18 Nov. 2009; published: 24 Dec. 2009

which demonstrates that A. morphnoides and A. weiskei occupy different fundamental niches. Additionally,
the finding that A. weiskei differs considerably from A. morphnoides in mtDNA and seems to be more closely
related to A. pennata than to A. morphnoides (Bunce et al. 2005; Lerner & Mindell 2005), prompted us to reevaluate morphological differentiation between these three taxa. Some sources (e.g. Gregory 2008a; BirdLife
International 2009; Gill et al. 2009) already treat weiskei as a separate species, based on the limited treatments
in Bunce et al. (2005) and Lerner and Mindell (2005), but a thorough analysis demonstrating the necessity ofthis taxonomic change has not previously been published.
Under the Biological Species Concept (BSC), guidelines as to how allopatric populations should betreated taxonomically have long been vague at best (Mayr & Ashlock 1991). Recently, Helbig et al. (2002)
worked out more detailed guidelines for assigning species rank. According to their criteria, allopatric taxa
should be considered species if they are fully diagnosable in each of several discrete or continuously varying
characters related to different functional contexts, e.g. structural features, plumage colours, vocalizations, orDNA sequences, or some combination of these, and when the sum of the character differences corresponds toor exceeds the level of divergence seen in related species that exist in sympatry.
The distribution of A. weiskei has been described as New Guinea except Vogelkop (Marchant & Higgins
1993; Debus 1994; Ferguson-Lees & Christie 2001, 2005). In addition to our observations, A. weiskei wasrecently reported (as Little Eagle) from the Arfak Mountains and Batanta Island (Gregory 2008b). It has been
considered as uncommon or rare and presumably resident in Wallacea (Coates & Bishop 1997). A small eagle
observed on Halmahera in July 1989 was considered to have been a Little Eagle, and was reported asapparently a vagrant from Australia or New Guinea (King 1990). Because of the confusion in the literature
about the geographic range of A. weiskei, we present our own observations of the species in the Moluccas andon western New Guinea.
Here we present results from evaluation of 12 morphological characters, published DNA sequences fromA. pennata, A. morphnoides and A. weiskei, and over 30 field observations of A. weiskei. From these new data
and further evaluation of published data, we conclude that A. morphnoides and A. weiskei should be regarded
as distinct species.

Material and methods
Morphological measurements and plumage studies were carried out at The Natural History Museum, Tring(BMNH). Measurements were taken using callipers and a ruler. A total of 25 specimens representing threetaxa (12 A. pennata, 8 A. morphnoides, 5 A. weiskei; see Appendix) were measured for 12 variables: flattened
wing length, Kipp’s distance = primary projection (distance from the tip of longest tertial to the wingtip), taillength (measured from the insertion of the central rectrices), length of hind toe, middle toe, hind claw, middle
claw, tarsus length, bill length from cere, bill breadth (measured immediately anterior to the cere), bill depth
and number of dark tail bands.
We used principal component analysis (PCA) to help explain mensural variability within and amongindividuals, sexes, and taxa. This multivariate method uses the correlation structure among variables to
identify underlying, independent principal components (PCs), which are linear functions of the originalvariables that may explain the dimensions associated with high data variability (see e.g. Afifi & Clark 1998).
The first few PCs typically explain most of the variation in the original data, and some operational meaning
can be attached to the PCs to aid in their interpretation. Our PCA analysis was based on a correlation matrix toallow scaling to unit variance.
The sex of most museum skins was given on the label; however, we also used a PCA to sex individuals.
We analyzed the PCA results by plotting bill length versus hind claw of individuals of each taxon, similar tothe predictive model used by Bortolotti (1984). No juveniles were measured because birds not fully grown
could add an age effect, and juveniles of many raptors have different feather lengths than adults. We
distinguished juvenile from adult rectrices by their narrower pale outermost tail bands and more pointed
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remiges. Because of small sample sizes, no statistical tests were performed to compare the three taxa withrespect to different morphological variables. Non-overlapping measurements are indicated in Table 1.
TABLE 1. Biometrics and differences between Pygmy Eagle Aquila weiskei, Little Eagle A. morphnoides and Booted
Eagle A. pennata. Non-overlapping measurements are indicated by an asterisk (*).
Morph.
Wing length
Kipp’s dist.
Tail length
Hind toe
Middle toe
Hind claw
Middle claw
Tarsus length
Bill length
Bill breadthBill depth
Tail bands

Sex

A. weiskei

A. morphnoides

A. pennata

n

min

max

n

M

2*

305

306

3

min
345

max
350

n
7

min
360

max
380

F

2*

315

340

5

384

390

5

385

408

M

2

72

105

3

95

100

7

85

120

F

2

111

123

5

95

140

5

100

120

M

2

163

165

3

163

173

7

190

200

F

2*

160

183

5

185

208

5

190

210

M

2

17.7

20

3

19.9

25.7

7

20

26.2

F

2

23.5

25.9

5

23.5

28.7

5

24

29.2

M

2*

37.9

39

3

41

43.6

7

38.5

42.7

F

2

42.2

42.8

5

42.5

49.8

5

41.7

48.3

M

2

25.8

27.3

3

26.1

27.8

7

23.7

27.4

F

2

31

31.2

5

28.8

33.6

5

27.3

30.8

M

2*

21.4

21.5

3

20.4

21.1

7

18

21.2

F

2

23.3

24.3

5

21.4

24.7

5

21.6

24.1

M

2

58

60

3

50

67

7

55

68

F

2

58

66

5

61

70

5

58

73

M

2*

25.6

28

3

29.8

31

7

28.4

30.9

F

2

30.3

32.1

5

31.5

34.3

5

30.6

33.6

M

2

10.8

11.2

3

11.5

12.6

7

9.9

12.5

F

2

12.6

12.9

5

12.4

14.7

5

11.8

13

M

2

15.6

16.3

3

16.3

16.4

7

15.2

16.9

F

2

18.2

18.3

5

17.3

19.1

5

17.1

18.5

M

2

7

8

3

7

8

7

0

6

F

2

8

8

5

7

8

5

0

7

JOG undertook field work for a total of 75 days on Halmahera, Ternate and Tidore in the Moluccas (7–8
Sep 1994, 1–25 Sep 1995, 30 Aug–17 Oct 1996) and 26 field days in the Arfak Mountains, Papua Barat(western New Guinea) (11–15 Sep 1994, 11–16 Oct 1995, 20–24 Mar 1996, 7–16 Jan 1997). Identification
criteria used in the field to distinguish A. weiskei are provided in the Discussion, under Field Identification.
We evaluated the levels of genetic differentiation among multiple individuals of A. morphnoides, A.
weiskei, and A. pennata and, for comparative purposes, other sister species pairs of booted eagles (subfamily
Aquilinae [Gadow 1893; Lerner & Mindell 2005] or tribe Aquilini [Amadon 1982; Helbig et al. 2005]), using
publicly available homologous DNA sequences from GenBank. Sequences were aligned in BioEdit andtrimmed to the shortest complete sequence (Hall 1999). Sister species pairs were selected based on results ofpublished studies (Haring et al. 2007; Helbig et al. 2005; Lerner & Mindell 2005) and verified by maximum
likelihood analysis in RaxML (Stamatakis et al. 2008). Uncorrected “p” distances and total base pairdifferences were calculated in Paup* v. 4.0b10 (Swofford 2004).

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Results
Biometry and plumage of Aquila weiskei, A. morphnoides and A. pennata
There was no overlap between A. weiskei and A. morphnoides in six characters (wing length in both sexes;
tail length in females; middle toe, middle claw, bill length and bill breadth in males; Table 1). Between A.
morphnoides and A. pennata there is no overlap in three characters (wing length, tail length and number of tailbands in males; Table 1).

FIGURE 1. Results of a Principal Component Analysis (PCA) of biometrics. 1 = Aquila weiskei, 2 = A. morphnoides, 3= A. pennata. Filled symbols indicate females; open symbols indicate males.

The PCA plot in Figure 1 indicates a taxon and sex separation of the individuals along the two firstprincipal components. The first principal component (PC1) explains 48% of the total variance, whereas thesecond component (PC2) explains 19% (Table 2). The sexes segregated without overlap on PC1, whereas thetaxa A. weiskei, A. morphnoides and A. pennata separated with very small overlap on PC2.
All three taxa have both a pale morph (most common) and a dark morph, with some intermediate
individuals. Major plumage differences between adults of A. weiskei, A. morphnoides and A. pennata aresummarized in Table 3 and illustrated in Figure 2. In adult plumage, Aquila weiskei has the upperparts darker
contrasting less with the paler scapular patch, and darker underparts with broader streaks than in either A.
morphnoides or A. pennata. Compared to A. morphnoides, weiskei lacks that species’ distinctive black crownwith a short crest that contrasts sharply to the paler nape. In A. weiskei the crown is dark brown streaked withno crest.
Four of the five specimens of A. weiskei at BMNH were pale morphs. The fifth was a dark morph
immature (shown in Figure 2). Eleven pale morph and one dark morph bird were observed in the field.
All pale morph A. weiskei at BMNH had all the underwing coverts whitish with small dark spots, similar
to light morph A. pennata. This pattern was also observed in eight of 11 pale morph birds observed in the fieldunder conditions allowing study. The other three pale morph individuals had underwing patterns very similar

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to A. morphnoides, with rufous brown lesser underwing coverts (leading edge) that contrast with the white
oblique band of the greater underwing coverts. Five of nine A. morphnoides at BMNH were pale morphs.
These pale morph specimens had all rufous-brown lesser underwing coverts contrasting with the white
oblique band of the greater underwing coverts (Figure 3). In the dark morph both A. weiskei and A.
morphnoides have all the underwing coverts dark.
TABLE 2. Loadings and total variance of the two first principal components for morphological characters of Pygmy
Eagle Aquila weiskei, Little Eagle A. morphnoides and Booted Eagle A. pennata.
Character

PC1

PC2

Wing length
Kipp’s dist.
Tail length
Hind toe
Middle toe
Hind claw
Middle claw
Tarsus length
Bill length
Bill breadthBill depth
Tail bands
Crest

-0.24
-0.21
-0.16
-0.29
-0.30
-0.35
-0.31
-0.22
-0.37
-0.32
-0.35
-0.18
-0.20

-0.44

Total variance

48%

19%

-0.52
-0.27
-0.17
0.22
0.23

0.15
0.48
0.23

Genetic differentiation
Homologous DNA sequences were available for 965 bps of cytochrome b (cyt-b) for two A. weiskei
individuals (AY987291.1, AY754045.1) and three individuals each of A. morphnoides (AY754044.1,
EU345503.1, AJ604499.1, AY987290.1), A. pennata pennata (EU345502.1, Y15760.1, AY987289.1) and A.
pennata dubia (AY987288.1, stored in GenBank under the synonym Hieraaetus pennatus minisculus
Y15761.1). Suspected contamination in the first ~240 bps of an additional A. pennata sequence, AJ604500.1,
precluded its use. Homologous sequence from 1041 bps of ND2 was available from a single representative ofeach taxon (A. weiskei, AY987113.1; A. morphnoides, AY987112.1; A. p. pennata AY987111.1; A. p. dubia
AY987110.1). The highest values for pairwise divergence were found between A. weiskei and A. morphnoides
and between A. weiskei and A. p. dubia (0.026, cyt-b; Table 4). Pairwise numbers of base pair differences
ranged from a minimum of three differences between the two subspecies of booted eagle to a maximum of 28
differences between A. morphnoides and A. weiskei (cyt-b, Table 4).
Cytochrome b sequences were available for 214 booted eagle individuals and seven outgroup individuals(Harpy Eagle Harpia harpyja, Crested Eagle Morphnus guianensis and Papuan Eagle Harpyopsis
novaeguineae). Sequences of length 965 bases were retained (n=128, including 6 outgroup sequences) andthree sequences of >924 bases or longer were included to increase taxon sampling (dataset available uponrequest). The topology of the maximum likelihood tree generated by these sequences corresponded topreviously published phylogenies, albeit with lower support values at deeper nodes (Helbig et al. 2005;
Lerner & Mindell 2005; and Haring et al. 2007; tree is available upon request). Sister species pairs discussed
here were all recovered with bootstrap support values of >90. Genetic divergence among sister species pairs
ranged from 1.7% between four individuals of Eastern Imperial Eagle Aquila heliaca and two individuals of
Spanish Imperial Eagle Aquila adalberti, to 4.7% between two individuals of Ornate Hawk-Eagle Spizaetus
ornatus and one Black and Chestnut Eagle Spizaetus isidori, with an average divergence of 2.7% (Table 5).

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FIGURE 2. BMNH specimens in dorsal and ventral view. From left to right, pale-morph male and dark-morph female
Aquila pennata, dark-morph male and light-morph female A. morphnoides, and dark-morph and pale-morph female A.
weiskei.

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FIGURE 3. Underside pattern of pale morph Aquila pennata (upper), A. morphnoides (middle) and A. weiskei (lower).
Drawn from photos and field sketches.

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TABLE 3. Main plumage differences between adult Pygmy Eagle Aquila weiskei, Little Eagle A. morphnoides andBooted Eagle A. pennata.
CharacterA. weiskei

A. morphnoides

A. pennata

Crowndark brown streaked

BlackishPale rufous to cream withfine black-brown shaftstreaks.

Crest

No crest

Short crest

No crest

Cheek

Dark streaked as rest of the head

Dark in contrast to paler nape Dark in contrast to rest ofand neck-side.
the head

Nape

Dark streaked as crown

Pale rufous to cream with Pale rufous to cream withnarrow shaft-streaks in narrow shaft-streaks
contrast to darker crown and without contrast to crownmantleBreast markings

Broad dark streaks

Narrow dark streaks

Narrow dark streaks

Upperwing coverts

Lack of distinctive light panels

Distinctive light panels

Distinctive light panels

Underwing
pattern in pale morph

Variable, from all coverts white Lesser underwing coverts A l l u n d e r w i n g c o v e r t s(80%, N=15), to lesser underwing rufous in contrast to white white
coverts rufous in contrast to white greater underwing coverts
greater coverts

MantleDark with little contrast to tertials Lighter than the dark tertials

Uppertail-coverts

Darker with little contrast to rump Pale buff to pale rufous in Pale buff to pale rufous incontrast to dark rumpcontrast to dark rump

Tail pattern

7–8 distinctive dark bars in less 7–8 distinctive dark bars
0–7 less distinctive dark
contrast to the light bars
in more contrast to the light bars
bars

Lighter than the dark
tertials

TABLE 4. Mitochondrial genetic divergence among Pygmy Eagles Aquila weiskei, Little Eagles A. morphnoides andBooted Eagles A. pennata. Number of base pair differences followed by uncorrected “p” distances in parentheses for1041 bps of ND2 above the diagonal for one individual of each taxon and for 965 bps cyt-b for two A. weiskei individuals
and three individuals of all other taxa below the diagonal.
A. weiskei

A. morphnoides

A. p. pennata

A. p. dubia

A. weiskei*

22 (0.022)

21 (0.021)

26 (0.026)

A. morphnoides

25–28 (0.026)*

20 (0.019)

23 (0.022)

A. p. pennata

23 (0.023)

22–25 (0.022)*

5 (0.005)

A. p. dubia

22 (0.024)

21–24 (0.023)

3 (0.003)*

Distribution
In the period 1994–1997, JOG observed Aquila weiskei 19 times in the Arfak Mountains, Vogelkop, New
Guinea, and 17 times in the North Moluccas (16 times on Halmahera and once on Ternate) (Table 6). Twelve
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of 36 total observations were under conditions that allowed descriptions of plumage. Observations of pairs
and territory display indicate that they are breeding in these areas, and are not vagrants as earlier suggested.
Figure 4 indicates the distribution of Aquila weiskei, A. morphnoides, A. pennata and the extinct giant Haast’s
Eagle A. moorei (based on Marchant & Higgins 1993; Ferguson-Lees & Christie 2005; Rasmussen &Anderton 2005; Naoroji 2006; own data).
TABLE 5. Pairwise genetic divergence between species of booted eagles. Uncorrected “p” distances for 965 bps cyt-b
calculated from all published sequences of booted eagle species pairs.
Sister species comparison (# of sequences)

Uncorrected Reference”p” distance

Nisaetus alboniger1 (3) vs. N. nipalensis (2)0.035

Lerner and Mindell (2005)

Nisaetus cirrhatus (3) vs. N. philippensis2 (1) 0.023

Helbig et al. (2005)

Spizaetus ornatus (2) vs. S. isidori (1)0.047

Lerner and Mindell (2005); Helbig et al. (2005); Haring
et al. (2007)

Aquila clanga (1) vs. A. pomarina (3)0.019

Lerner and Mindell (2005); Helbig et al. (2005)

Aquila audax (4) vs. A. gurneyi (1)0.028

Lerner and Mindell (2005); Haring et al. (2007)

Aquila fasciata (29) vs. A. spilogaster (5)0.018

Lerner and Mindell (2005); Helbig et al. (2005)

Aquila heliaca (4) vs. A. adalberti (1)0.017

Helbig et al. (2005)

Average distance

0.027

1

may be more closely related to Javan Hawk-Eagle Nisaetus bartelsi (Haring et al. 2007), a taxon not included in Lernerand Mindell (2005) or here.
2
may be more closely related to Southern Philippine Hawk-Eagle Nisaetus pinskeri (Haring et al. 2007), a taxon notincluded in Helbig et al. (2005) or here.

Discussion
Taxonomy
On the basis of the large number of diagnostic differences between Aquila weiskei and A. morphnoides,
we recommend that A. weiskei should be treated as a distinct species, and propose to use the English name‘Pygmy Eagle’ because it is the smallest eagle in the world, males being smaller even than the Little Eagle.
Parry (2001) used the alternative name ‘Weiske’s Eagle’ after the collector Emil Weiske, who collected thetype specimen in Astrolabe Mountains in Jun 1898, but we prefer to use the more descriptive ‘Pygmy Eagle’.
Other English names in use for weiskei include ‘Papuan Booted Eagle’ (Debus et al. 2007), ‘Papuan Little
Eagle’ (Gregory 2008a), and ‘New Guinea Hawk-Eagle’ (Gill et al. 2009), but we prefer the less cumbersome‘Pygmy Eagle’, which is now used by the Global Raptor Information Network (2009). Also, use of either of‘Papuan Booted Eagle’ or ‘Papuan Little Eagle’ would likely require modification of existing, wellestablished English names.
In regard to proposed assignment of species rank to allopatric taxa, Helbig et al. (2002) recommended thattaxa be considered species if they are fully diagnosable in each of several discrete or continuously varying
characters related to different functional contexts, e.g. structural features, plumage colours and DNA
sequences. We find that the Pygmy Eagle is diagnosable from the Little Eagle in several structural features(wing, tail, middle toe, middle claw and bill length; Table 1), 10 adult plumage differences (Table 3) and in
DNA sequences (Table 4; Bunce et al. 2005; Lerner & Mindell 2005).
We did not study the Pygmy Eagle in juvenile plumage. In the Little Eagle juveniles are similar to adultbirds, except for being richer rufous on the head and underparts, lacking black patches on the crown, andhaving thinner dark streaks on the underparts. In fresh plumage, the upperparts are darker than in adults, with

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less contrasting pale scapular patch and diagonal band across inner wing coverts. Juvenile Little Eagles are
therefore probably more similar to A. weiskei than are adults. Differences between juvenile and adult Little
Eagles are negligible in worn plumage (Marchant & Higgins 1993).
TABLE 6. Observations of Pygmy Eagle by JOG in the Arfak Mts, western New Guinea (1°13’S 134°03’E), andHalmahera (1°05’N 127°32’E) and Ternate (0°50’N 127°22’E) in North Moluccas in 1994–1997. S = soaring, D =undulating display flight, H = hunting.
Locality

Altitude (m)DateArfak Mts

1600-1700

14.09. 1994

Arfak Mts

100-200

11.10. 1995

Arfak Mts

100-200

Arfak Mts

Time

Number of individuals

Behaviour

1

S

12.40

1

S

11.10. 1995

13.50

1

S

200-300

12.10. 1995

08.20

1

S

Arfak Mts

400-500

13.10. 1995

06.40

1

H

Arfak Mts

700-800

15.10. 1995

11.50

1

S

Arfak Mts

700-800

16.10. 1995

07.39

1

S

Arfak Mts

700-800

16.10. 1995

08.15

2

S

Arfak Mts

700-800

16.10. 1995

08.39

1

S

Arfak Mts

700-800

16.10. 1995

08.47

1

S

Arfak Mts

700-800

16.10. 1995

08.59

1

S

Arfak Mts

100-200

20.03. 1996

2

S

Arfak Mts

300-400

24.03. 1996

1

S

Arfak Mts

1850-1950

11.01. 1997

1

S

Arfak Mts

1600-1700

13.01. 1997

1

S

Arfak Mts

1600-1700

13.01. 1997

09.35

1

S

Arfak Mts

1750-1850

13.01. 1997

11.00

1

S

Halmahera

200-300

30.08. 1996

1

S

Halmahera

200-300

01.09. 1996

1

S

Halmahera

200-300

02.09. 1996

1

S

Halmahera

300-400

03.09. 1996

10.23

1

S

Halmahera

200-300

03.09. 1996

12.00

1

D

Halmahera

300-400

03.09. 1996

14.30

1

S

Halmahera

300-400

08.09. 1996

11.20

1

S

Halmahera

300-400

08.09. 1996

11.48

1

D

Halmahera

300-400

09.09. 1996

10.17

1

S

Halmahera

300-400

13.09. 1996

14.50

1

D

Halmahera

100-200

16.09. 1996

09.00

1

S

Halmahera

200-300

30.09. 1996

10.10

1

S

Halmahera

200-300

30.09. 1996

11.54

1

D

Halmahera

100-200

02.10. 1996

12.35

1

H

Halmahera

300-400

02.10. 1996

15.52

1

S

Halmahera

200-300

04.10. 1996

12.20

1

H

Ternate100-200

11.10. 1996

12.30

1

S

Dark morph Pygmy Eagles seem to be much rarer than pale morphs (Ferguson-Lees & Christie 2005; thisstudy). It is unknown if the ratio of the two morphs shows geographic variation. In the Little Eagle the pale
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Zootaxa 2326 © 2009 Magnolia Press · 33

morph is generally more common than the dark morph but the ratio varies. The dark morph predominates or isequally common to the pale morph in south-eastern Queensland, whereas in West Australia the pale morph is
15 times as common as the dark morph (Marchant & Higgins 1993). In the Booted Eagle, the pale morph ismore common than the dark morph, with 80% light morphs in Spain and South Africa (n=648) and 60% palemorphs in Greece and the Middle East (n=362; Forsman 1999; Ferguson-Lees & Christie 2001).
The plumage differences between the Pygmy and Little eagles are greater than those between the Little
and Booted eagles, which are widely recognized as distinct species. We therefore consider that the large
number of diagnosable characters indicates that Pygmy Eagle is a distinct species, regardless of which species
concept is used. The large size difference between Pygmy and Little eagles (female Pygmy Eagles being
similar in size to Little Eagle males) would probably function as a reproductive barrier should they meet. In
addition, the size difference between the two taxa may function as an ecological isolation mechanism, asintermediate-sized hybrids could be less well-adapted.
Pygmy and Little eagles diverged from Booted Eagle or a common ancestor about one million years ago(2.2–2.6% divergence in mitochondrial cyt-b and ND2). Western Palearctic populations of Booted Eagle are
migratory, wintering in Africa. In South Africa the resident subspecies dubia has evolved (Brooke & Vernon
1981). Eastern Palearctic populations of Booted Eagle are also migratory and winter in peninsular India.
Vagrants have been observed southeast to peninsular Malaysia, Sumatra, Java and Bali (Ferguson-Lees &Christie 2001). Such vagrants of Booted Eagle or a common ancestor may have reached New Guinea andAustralia historically and diverged over time, leading to the present-day Pygmy Eagle and Little Eagle.
Interestingly, a third closely related species evolved at approximately the same time, the extinct giant Haast’s
Eagle Aquila (Harpagornis) moorei in New Zealand, as it has a similar level of genetic divergence from the
Booted Eagle as from the other two taxa (Bunce et al. 2005).
Field identification
In the Moluccas the possibility of misidentification of Pygmy Eagles with vagrant Booted Eagles exists.
Of the 15 pale morph Pygmy Eagles studied in the field and at BMNH, 12 (80%) had all underwing coverts
white, lacking the contrasting white band between the rufous lesser underwing coverts and the dark
secondaries found in all pale morph Little Eagles. All of the several hundred pale morph Little Eagles seen (inphotos, museums and in the field) were all of this normal pale morph (Stephen Debus pers. comm.). The
different underwing pattern of the Pygmy Eagle in the field is what first caught JOG’s attention, as thisplumage has not previously been described. Although the Pygmy Eagle’s underwing pattern is broadly similar
to that of pale morph Booted Eagle, it is easily distinguished under good observation conditions from thepattern in the Booted Eagle by the much paler inner parts of the outer primaries and very pale inner primaries.
This produces a large pale patch on the hand that contrasts with the dark barred outer primaries andsecondaries of the Booted Eagle. The Booted Eagle has black unbarred tips of the outer flight feathers (Figure3). The dark morph Pygmy Eagle is more similar to the Booted Eagle in underwing pattern, but can be
distinguished by the less contrasting upperwing pattern and the lack of pale-coloured contrasting uppertail
coverts. Booted Eagles also have characteristic white shoulder-patches visible both in flight and whenperched, lacking in Pygmy Eagle.
Juvenile Brahminy Kites Haliastur indus (Boddaert, 1783) have a similar plumage to the pale morph
Pygmy Eagle both on the upperside and underside. However, under good observation conditions the kite isreadily distinguished by its broader wings with an S-curved trailing-edge, shorter unbarred tail, bare tarsi andblack unbarred primary-fingers.
In New Guinea the Pygmy Eagle may be confused with the Whistling Kite Haliastur sphenurus (Vieillot,
1818). Under good observation conditions the kite should easily be distinguished by its more slender body andlonger rounded tail. Dark morph Pygmy Eagles can also be confused with Black Kites Milvus migrans(Boddaert, 1783); however, the kite’s deeply forked tail shape is normally diagnostic.

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Genetic differentiation
The level of mitochondrial sequence divergence between A. morphnoides and A. weiskei (2.2–2.6%; Table4) lies within the range of that seen for seven other sister species of booted eagles, falling just below theaverage value of 2.7% (Table 5). Evidence of multiple fixed nucleotide differences between A. weiskei and A.
morphnoides further supports a history of genetic isolation for these taxa.

FIGURE 4. Distributions of Aquila weiskei, A. morphnoides and A. pennata (modified after Marchant & Higgins 1993;
Ferguson-Lees & Christie 2005; Rasmussen & Anderton 2005; Naoroji 2006; own data). Light blue indicates the winterrange of A. pennata. The probable distribution of the extinct Haast’s Eagle Aquila moorei is also illustrated.

Distribution
The distribution of the Pygmy Eagle has been described as the mountain forests of central and eastern New
Guinea (Condon & Amadon 1954); as sparsely distributed throughout New Guinea (no records from
Vogelkop), but also as common but rather inconspicuous, from sea level to 1950 m (Beehler et al. 1986); andas uncommon or rare in Wallacea; presumably resident (Coates & Bishop 1997). A small eagle observed onHalmahera on 27–28 Jul 1989 was considered to have been a Little Eagle, and was regarded as apparently avagrant from Australia or New Guinea (King 1990). Another small eagle, considered to be a juvenile Little
Eagle, was observed in flight over primary forest at c. 700 m in Feb 1996 on Seram (Coates & Bishop 1997).

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Zootaxa 2326 © 2009 Magnolia Press · 35

Neither of these was distinguished (sub)specifically at the time, but would surely have been weiskei rather
than morphnoides. The distribution of weiskei was described as New Guinea except Vogelkop in Debus(1994). Elsewhere (Marchant & Higgins 1993; Ferguson-Lees & Christie 2001, 2005), the distribution is
described as a more restricted range across New Guinea on each side of the central ridge, up to 2100 m except
Vogelkop. In addition to our observations, weiskei was recently reported (as Little Eagle) from the Arfak
Mountains and Batanta Island (Gregory 2008b). This species may have been previously overlooked becauseof similarities with Whistling Kites and juvenile Brahminy Kites. Our observations from Vogelkop and the
Moluccas indicate that the Pygmy Eagle is resident in these areas.

Acknowledgments
We thank Robert P. Prys-Jones at The Natural History Museum (BMNH), Tring, for permission to take photosof skins, and Mark P. Adams and Nigel Cleere for help with the skin collection; copyright remains with
BMNH. We thank Nils Røv for his invaluable efforts during field work together with JOG. We thank Stephen
Debus for information about the Little Eagle and for comments on the manuscript. We thank the reviewers
George Sangster and Bruce Beehler for valuable corrections and comments. We thank Kari Sivertsen fordrawing the map and Trond Haugskott for painting the eagles in Figure 3.

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Appendix. Specimens of Aquila (Hieraaetus) examined in this study
Aquila weiskei: BMNH 1901.4.28.5, adult female, Astrolabe Mts., SE New Guinea (type specimen, collected by Emil
Weiske); BMNH 1911.12.20.434, unsexed immature, Utakura river, Dutch New Guinea; BMNH 1955.6.N.20.357,
adult female, Astrolabe Mts., SE New Guinea; BMNH 1913.3.6.35, female, Moroka Mts., British New Guinea;
BMNH 1882.3.8.45, unsexed adult, Astrolabe Mts., SE New Guinea.
Aquila morphnoides: BMNH 1955.6.N.20.353, adult female, Swan River, Australia; BMNH 1955.6.N.20.354, adultfemale, Queensland, Australia; BMNH 1955.6.N.20.355, adult male, NSW, Australia; BMNH 1955.6.N.20.356,
unsexed pullus, NSW, Australia; BMNH 1874.1.12.18, unsexed adult, Queensland, Australia; BMNH 1875.4.20.11,
unsexed adult, Queensland, Australia; BMNH 1925.11.1.276, adult male, Queensland, Australia; BMNH 1969.4.22,
adult female, Grant range, NE Australia; BMNH 1906.12.20.11, adult male, W. Australia.
Aquila pennata: BMNH 1885.8.19.1308, female, India; BMNH 1894.6.16.295, male, Egypt; BMNH 1875.11.22.17,
female, Granada, Italy; BMNH 1885.8.19.1300, juvenile female, India; BMNH 1885.8.19.1297, male, India;
BMNH 1887.11.1.319, female, India; BMNH 1885.8.19.1299, adult male, India; BMNH 1885.8.19.1302, male,India; BMNH 1885.8.19.1295, female, India; BMNH 1940.12.1.57, female, Beograd, Serbia; BMNH
1875.11.22.18, adult male, Granada, Italy; BMNH 1901.7.17.6, unsexed adult, Sevilla, Spain; BMNH
1875.11.22.21, adult male, Sevilla, Spain; BMNH 1955.6.N.20.338, adult male, France; BMNH 1945.20.5, adultmale, Crete, Greece; BMNH 1886.3.25.125, male, India.

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