First Symposium on Steller’s and White-tailed Sea Eagles in East Asia pp. 1-10, 2000
UETA, M. & MCGRADY, M.J. (eds) Wild Bird Society of Japan, Tokyo JapanThe morphology of the bill apparatus in the Steller’s Sea Eagle
Alexander Ladyguin
Moscow State University, Science Park, Educational-Research Center "Ecosoil", 119899, Moscow, Russia.
e-mail: ladygin@rc.ru

INTRODUCTION
The osteology of birds has been more thoroughly investigated than any other anatomicalsystem in that group of animals. Being made up of many individual parts, each with numerous
details, the skeleton offers more evident possibilities for study than other systems. Bones also
make up most of the fossil evidence.
The avian skull and mandible have featured largely in systematic accounts (e.g. Huxley
1867, Pycraft 1898, Barnikol 1952, Fisher 1944), and have attracted much attention as tofunction (e.g. Lakjer 1926, Hofer 1950, Bock 1970, Dzerdjinskiy 1972, 1986, Hertel 1994).
Study of the avian skull rests in part on research during developmental stages, as in the adultfew suture lines are retained. Some phases of development have been investigated by Parker
1890, Marinelly 1936, Jollie 1957, Sushkin 1899, 1902. Some species of Falconiformes were
investigated by Parker 1873, Sushkin 1899, Fisher 1944, Jollie 1977, Hertel 1994, Ladygin
1994. No detailed investigation of the skull of sea eagles has been undertaken.

MATERIAL AND METHODS
Twenty-one skulls of Steller’s Sea Eagle Haliaeetus pelagicus, 12 skulls of White-tailed
Sea Eagle H. albicilla and 18 skulls of Bald Eagle H. leucocephalus were examined. Fiveadult, 4 subadult and 3 eaglet (4 weeks old) skulls were included in the myology study.
Jaw muscles of three adult Steller’s Sea Eagle and one adult White-tailed Sea Eagle weredissected. I drew original illustrations in pencil, using a dissecting microscope and drawing
scale tools for all anatomical figures, final figures were inked.
The figures have been sketched directly from the specimens; they are not designed topresent exact dimensions or proportions. The figures give an impressions of the particular
structure described and any significant relationships which it might have with other structures.
Measurements (Table 1, Fig. 1) were taken with digital calipers in millimeters, and all
analysis was made using the SYSTAT statistical package.
Specimens studied were housed at: Zoological Museum of the Moscow State University,
Vertebrate Department of the Biology Faculty of the Moscow State University, ZoologicalMuseum of the Zoological Institution of St. Petersburg, American Museum of Natural History,

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Table 1. Sea eagle skull measurements.
Cranium
Total length = length from
occiput to tip of beak instraight line (Fig. 1, A)

Beak
Total length = length fromnaso-frontal hinge to tip ofbeak

Mandible
Total length = maximum
length along ramus

Total width = maximum skull
width across postorbital
processes

Tomial length = length fromanterior part of nares to tip ofbeak

Maximum width = width in
the region of articulation

Occipital width = maximum
width of occipital region

Tomial width = width atanterior part of nares

Articular length = length fromarticular to tip of mandible

Maximum height = maximum
height of the base of the
cranium (Fig. 1, B)

Tomial depth = depth atanterior part of nares
Tomial cord = length fromanterior part of nares to tip ofthe beak (Fig. 1, D)
Tomial arch = curved lengthfrom anterior part of nares to
tip of the beakTomial arch radius = lengthfrom cord to point of maximum
curvature (Fig. 1, E)
Maximum height = height at
the anterior part of nares

All anatomical materials used in this study were obtained from birds killed by accident .

A

B

C

E
D

Fig. 1. Measurements used to calculate cranial indexes.

Zoological Museum of the Zoological Institution of Halle (Germany), Paleontological
Institution, private collections.

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Table 2. Morphological characteristics ofSteller's Sea Eagle skulls (N = 15).
Characteristics
Length of skull
Width of skull
Height of skull
Length of cranium
Length of the upper jaw
Length of the low jaw
Height of the upper jaw
Width of the upper jaw
Width of the lower jaw
Diameter of the orbital

Average length± SD (mm)146.0 ± 27.8
71.0 ± 9.5
40.0 ± 8.0
69.5 ± 12.0
82.0 ± 9.5
117.0 ± 11.5
35.0 ± 2.0
29.5 ± 6.5
70.2 ± 5.4
35.5 ± 9.9

Phylogenetic remarks
Steller’s Sea Eagle is a member of
the Genera Haliaeetus, Family Accipitridae,
Order Falconiformes, Class Aves.
This is a small group of large
Accipitrids, that feed mostly fish andincludes such well studied birds as theBald Eagle and White-tailed Sea Eagle.

Definition and orientation
Basically, the head skeleton can bedivided into three parts: cranium (or, morecorrectly, neirocranium), primary upper
jaw and lower jaw. These three elements are connected to one another by a complex ofbones, ligaments and muscles. We will considered these elements of the skull separately, andthen discuss the interactions between them that allows them to work as a single biomechanism.

RESULTS AND DISCUSSION
General description
The skull of the Steller’s Sea Eagle is one of the most massive among all Accipitrids,
including griffons (Gyps sp.) and other vultures (Table 2).
Cranium
The cranium is the large, durable capsule that protects the brain, eyes, and hearing
capsules. The cranium provides a stable base for the connection of most ligaments and jaw’s
muscles. The Steller’s Sea Eagle’s cranium is very hard, all bones are well developed, andmost cranium foramens, as is typical in other birds of prey, are absent.
Viewed from above the skull of the Steller’s Sea Eagle is elongated-triangular with arounded base. The cranium is usually flattened above and rounded behind. It has distinct
cerebral bulges and a shallow median gutter. The median depression may extend forward andbecome accentuated in the brow region. The cranium is laterally widened. The ear area iseasily characterized. The depressor mandibulae has an extensive origin beginning lateral andposterior to the squamosal articulation of the quadrate (Fig. 2).
The cranium of Steller’s Sea Eagle has some characteristic processes. The zygomatic
process, to which the Musculus adductor mandibulae externus is connected, is short andblunt. The articular process is exposed at the edge of the tympanic cavity. It is large and has
a distinct, lateral ridge, which makes it triangular in cross-section. The base of the skull is
inflated. The parasphenoid bulges ventrally either side of the midline. The occipital condyle isexceptionally wide, and has median, posterior and ventral grooves. The basiparasphenoid area

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Pr. postorbitalis

superciliare praefrontale

nasalenostrils
maxillarePr. zygomaticus

palatinum
occipitale
tomiumpterigoideum
jugale Pr. palatinum
quadratumpremaxillaris
mezethmoideum
lower jaw
Fig. 2. Lateral view of Steller's Sea Eagle skull.

is triangular. The basiparasphenoid processes are low, rounded ridges extending out to eitherside. They are separated medially by a small median basyparasphenoid process. Posterior to
the latter and anterior to the condyle, the basioccipital bone has the form of a low ridge
instead of a pit.
The tympanic articulations are well developed, the Eustachian tubes are partly open
and the anterior wall of the anterior tympanic recess has a blunt, hornlike alaparasphenoid
process. The basipterigoid processes are developed only in the chicks and disappear in theadult birds.
The basitympanic process width, the distance between the lateral points of thebasiparasphenoid plate, ranges from the 55 to 65% of the postemporal width, measured acrossthe skull at the zygomatic processes.,This is the widest among all Accipitrids.
Viewed posteriorly, the tympanic margin has a characteristic sigmoid shape. Viewed
laterally, this margin is accutely angled at the junction of the squamosal and exoccipital. A
posterior tympanic process occurs in Steller’s Sea Eagle, but involves an outgrowth of thedorsal rim.
The lateral basiparasphenoidal processes are low rounded lumps showing variously
scrawled, minor ridges or tubercles. The exoccipital process is a somewhat triangular, bluntlytipped, ventro-anterior projection at the ventral margin of the tympanic cavity.
The interorbital septum is not perforated in adult birds, which is unusual for birds of preyand may suggest a relatively small orbit. Normally, within the limit of space in the bird’s skull,
the large orbits crowd out less important structures, and the interorbital septum in many
raptorial species is quite narrow and even perforated. Perforation is attenuated the bones andskull as a whole, but in Steller’s case probably, the skull as a whole must be more secure andthis reason dictates any bones to be faster.

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The olfactory capsule, which encloses the olfactory nerve, is narrow. The palatine
processes of the maxillae are well separated, and approach those of the desmognathous palate.
The prefrontal is well developed and is free. It is connected via sindesmos with thefrontal and the lateral side of ethmoid. The supraorbital process is broad with a truncated tip.
The posterior margin is squared for the reception of the small nasal gland, which lies between
the prefrontal and the orbit margin. The superciliare form is a small cycle-like bone attached tothe posterior edge of the supraorbital process.
The orbital process of the prefrontal is free, but in part supported by the lateralmezethmoid plate. This robust processes widens out into a somewhat spatulate ending; thedistal portion is elongated (correlated with the deepening of the upper mandible). The ventral
caudal ends of these processes are in contact with the medial part of the mezethmoid, beenhang upon, but not fused with it.
The vomer is formed of lateral components. This bone is a thin, laterally compressed,
splint lying in the septum between the internal nares. The anterior end exhibits small reduction
in size. The tip of the vomer is free. The vomer appears to be rooted to the rostrumparasphenoid by fibers. The vomer is associated with the anteropterigoids h, in the adult, areindistinguishably fused to the palatines. The vomer appears to be rooted to theanteropterygoids by fibers.
The final element of the palate,the pterigoid, articulates antero-medially with the palatine
and posterolaterally with the quadrate. The anterior articulation of the pterigoid, as viewed
ventrally, is a straight line. The pterygoid process is formed from the anteropterygoid, which
is fused with the palatine. The palatine fossa is well excavated and is bounded medially by astrongly developed choanal ridge.
When viewed from below, the quadrate, one of the most complicated of skull bones, has
a distinctly triangular shape, the base is forward, and the outer side shortest. Seen fromabove, the eagle quadrate has a shallow, rounded notch on its outer posterior margin, and theinner process juts straight medially (protrude straight inward) as part of the relatively straight
posterior margin
The Steller’s quadrate has a large, long and wide Pr. orbitalis, which starts from themedial part of the bone’s corpus and is directed inside and forward. The lateral surface of thiswell developed process provides space for the large Musculus adductor mandibulae posterior.
A pneumatic foramen is found on the dorsal medial surface of this inner process. The
cavity of the articular portion is continuous with the tympanic cavity through the foramen.
Upper jaw
The bill in adult sea eagles is bright yellow, unlike those of most other adult raptors; inthe subadult the bill is brownish with some yellow lines. The upper jaw has a very sharp, long
hooked beak.
The Steller’s Sea Eagle bill is unusually long for Accipitrids (but typical for sea eagles)

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Table 3. Average ( ± SD) of cranial indices in Steller's (N = 15) and White-tailed Sea
Eagles (N = 12).
Index letter
IndexesA
Length of lower jaw symphysis / maximum
length of the lower jaw
B
Diameter of the orbit/length of the cranium
C
Maximum height of the upper jaw / maximum
height of the cranium
D
Width/height of the upper jaw
ELength of the upper jaw/length of the skull
F
Length of the upper jaw / length of the
cranium

Steller's
0.23 ± 0.06

White-tailed
0.20 ± 0.05

0.47 ± 0.07
0.87 ± 0.10

0.41 ± 0.05
0.55 ± 0.12

0.36 ± 0.09
0.55 ± 0.09
1.20 ± 0.01

0.40 ± 0.08
0.46 ± 0.08
0.70 ± 0.02

(cranial index F = 1.2, Table 3), and very deep, even for sea eagles (cranial index C = 0.87,
Table 3). The upper jaw is even longer then the cranium, and has about the same depth. Suchunusual proportions are not found in other Falconiformes.
The shape of the upper part of the bill above the nares is large, inflated and atypicalarched. This area of the skull is quite important because the cranium and beak are connected
here and this articulation is subjected to large amounts of force during feeding. The arched,bulbous form this area exhibits in cross section provides structural strength necessary when
during feeding.
The narial aperture is usually triangular-ovoid. The external naris is located in theventroanterior portion of the lateral wall of the vestibular chamber. Anteriorly the vestibule iswell ossified, although a gap or small fissure usually occurs between it and the premaxilla.
Posteriorly the wall is incomplete below the median naris. The lateral margin of the mediannaris is essentially part of the ventral process of the nasal and dorsal part of nasal process of themaxilla.
The bony nasal septum extends back to the craniofacial gap. It is normally not perforated
or it has has irregular perforations; the membranous nasal septum is never perforated.
The frontonasal hinge is simple. The prefrontal bones (articulate) directly, without any
overlap against the lateral edge of the frontal and nasals. This contact, when viewed dorsally,
creates a straight anterio-posterior line.
The rostrum-labial bar hinge is characterized by the long pointed spine of the premaxilla
which projects posteriorly along the labial bar and the slim anterior tip of the jugal.
The palatal is desmognathous - well developed palatine processes contacts each other andare completely fused. This characteristic also might serve to reinforce the Steller’s skull
against lateral forces that must occur during the ripping of food items.
The palatal aspect of the rostrum (as in all Accipitrids) has the tip of the praemaxilla
braced by ossification of the ventral margin of the nasal septum. This is called the septal bar.
The palatal vacuity is vestigial and divided by this bar. In Steller's Sea Eagle the contact andfusion of these maxillary processes has crowded out the septal bar, and it is reduced it to a7fibrous vestige.
The palatal surface of the tip of therostrum has a characteristic median grove
formed by the difference in the levels of thePr. palatinumpremaxillaris
palatine processes of the premaxilla and thevomerseptal bar. In this grove lies a median ridge
palatinum
formed by the septal bar.
The tomium is straight, not toothed. The
praefrontale
tomial margin of the premaxilla is deeply
jugalegrooved in the region of the hook. Thesepterigoideum
grooves appear to be sites of growth for the tip
quadratumof the horny sheath. The tomial margin of thepremaxilla is posterior to this groove. Viewed
laterally, it presents an even, somewhat
basitemporale
sigmoid, curved outline.
Foramen occipitale
The posterior palatal process of theFig. 3. Ventral view of Steller's Sea Eagle skull,premaxilla is weakly developed. It lies lateral
ventral view.
to the anterior end of the palatine. The jugale
is a long narrow bone, and connects the lateroventral end of the beak to the lateral part of the quadrate. There are no specific characteristicsof the jugale in the Steller’s Sea Eagle.
maxillare

Lower jaw
When viewed from above, the lower jaw is strongly triangular. It is relatively long andstrong. In comparison with other Accipitrids; the Steller’s lower jaw is relatively deep and has
a well developed lateral surface, where powerful skull muscles find ample space for attachment.
Skull ligaments
The ligamental apparatus (Fig. 3) of the Steller’s skull is very typical for birds.
The quadrato-maxillae articulation is regulated by three ligaments: Ligamentum jugomandibulare internum, Lig. jugo-mandibulare externum and Lig. postorbitale. The
Ligamentum jugo-mandibulare internum which connects the caudal edge of the jugal to thecaudal edge of the lower jaw after turning behind the jaw articulation, attached to the caudaledge of the lower jaw
The Lig. jugo-mandibulare externum connects the caudal edge of the jugale and thelateral surface of the lower jaw just anterior to the jaw articulation.
The Lig. postorbitale connects the postorbitalis processes of the cranium and the lateralsurface of the lower jaw.
All three ligaments are very strong and short in comparison to other birds of prey.

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aponevrosys lateralis
Ligamentum
postorbitale
Musculus adductor
mandibulae posterior
M. depressor
mandibulae
aponevrosys major
M. adductor
mandibulae
externus

L. jugo-mandibulare
externumM. pterygoideus
L. jugo-mandibulareinternumFig. 4. Ligaments and the muscles on the lateral surface of the Steller's Sea Eagle skull.

The Lig. occipito-mandibulare is covered by the M. depressor mandibulae. It connects
the caudal edge of the lower jaw to the ventral surface of the cranium. This ligament is alsoquite strong, but is relatively long.
The head muscles
Steller’s Sea Eagle head muscles are very powerful (Fig. 4).
The Musculus adductor mandibulae externus is well developed and covers a wide area on
the lateral surface of the mandible. This is the most important and well-developed skull
muscle. It lifts the lower jaw and clamps upper and lower jaws together. For this task thismuscle in Steller’s eagle fills a large areafrom the jaw’s articulation to the mouth This is not
the right word, but I don’t know what you want to saycorner.
The M. pterygoideus is also well developed and also covers a large area on the lateralsurface of the lower jaw just under the end of the M. depressor mandibulae.
In most Accipitrids this muscle is less developed and does not occupy the whole of thelateral surface. M. pterygoideus in cooperation with the Musculus adductor mandibulae
externus depresses the upper jaw, and works with that muscle to clamp the jaws together.
Given the way in which sea eagles feed, (fixing a food item with the foot and tearing
relativelysmall pieces from it) this muscle is quite important because it fit reinforces the upper
jaw against untucking.
Depressor mandible is a muscle of great interest. The origin of this muscle is restricted
to the tympanic rim and caudal surface of the cranium. The insertion of this muscle is on thecaudal surface of the mandible. Depressor mandible opens the jaws. It is well developed incomparison with other Accipitrids and probably adds to the general reinforcement of Steller’sskull.
Other muscles are morphologically similar to those found in other Accipitrids.

9
The hyoid apparatus
The basyhyal is round (or slightly flattened above and rounded below.) in cross-section
and lacks any indication of a keel. The enthoglossal ossification is composed of elongated rods
with articulation areas at their mid-points. Generally, the hyoid apparatus in Steller’s issimilar to that of most Accipitrids. It is relatively undeveloped and delicate (Ladygin 1994).

CONCLUSION
From this survey of features, the Steller’s Sea Eagle type can be characterized as follows:
The desmognathous palate is formed by the palatal processes of the maxillae meeting at
the middle, behind and below an ossification of the ventral margin of the nasal septum which
extends forward to the tip of the bill. The vomer is a vertical midline plate, which is bilateralposteriorly. The nasal vestibule is partly ossified forming a median perforated septum andpartial walls in anterioriorly and ventrally. The narial opening, framed by the nasal bone, isroughly triangular in shape and obstructed by ossification in the lateral vestibular wall. The
rostrum is large and strongly hooked; the premaxillary and narial portions are of about equal
length. The fronto-nasal hinge is simple; the prefrontal is well developed, free and has asuperciliary bone associated with its supraorbital processes. The cranium is strongly contoured
with cerebral bulges, a median groove and grooves above the orbital margins; the orbital
margins are only slightly extended. The zygomatic and articular processes of the squamosal are
well developed. The basipterigoid processes are vestigial in the adult, and the mandibulae lacka posterior fenestra. The basyhial is not keeled. The upper jaw in Steller’s eagle isextraordinarily massive when compared to those of other sea eagles, its depth accounting
greatly for the massive appearance.
Birds jaws are powerful ‘tools’ used for feeding, especially in the birds of prey. The
Steller’s eagle’s strong, very curved bill is the perfect implement for food riping and tearing
large carcasses into small pieces that are easy to swallow. The main food of Steller’s eagle arelarge fish, sometimes weighing about 6-7 kilograms, similar to the eagle’s own mass. In the
Bering and Ochotskoe seas the main fish species upon which they feed are the anadromous
salmon. Fish skin is tough and difficult to tear. But field observations suggest that Steller’s
Sea Eagles can consume about 900 g of fish in 3-4 minutes. In comparison, White-tailed Sea
Eagle feeding at the same locations spend about 18 minutes to consume the same amount, andGolden Eaglerequires 28 minutes (Ladygin 1994, 1996).
Field research in Kamchatka (Ladygin 1992, 1994, 1997) has shown that Steller7s SEa
Eagle occur in large aggregations on the wintering grounds, and at these aggregations many
aggressive interactions between tens of eagles occur during feeding, behavior that is similar togroups of vultures feeding on a single carcasse. In the case of the Steller’s Sea Eagle strong
competition between individuals exists at feeding places during wintertime. The timenecessary for food preparation can be a key factor in survival. Because of this, morphologicalstructures that can reduced time of feeding should be considered adaptive and are promoted.

10

LITERATURE CITED
Barnikol, A. 1952. Morphology of Nervus trigeminus in birds with special attention on Accipitridae,
Cathartidae, Striges, Anseriformes. J. cognitive zoology 157 (Supplement): 285-332. In German.
Bock, W.J. 1970. Affinities between some avian orders based upon their cranial morphology. XV Congr.
Intern. Orn. Abst. p. 66.
Dzerdjinskiy, F. Ya. 1972. Biomechanical Analysis of the Bird’s Bill Apparatus. Nauka, Moscow. In
Russian.
Dzerdjinskiy, F. Ya. 1986. Some structural correlates of active upper jaw protraction in birds. XIX Congr. Int.
Orn., Ottawa, Canada, 22-26 June 1986., Abst. p. 507.
Fisher, H.I. 1944. The skulls of Cathartid Vultures. Condor 46: 272-296.
Hertel, F. 1994. Diversity in body size and feeding morphology within past and present vulture assembleges.
Ecology 75: 1074-1084.
Hofer, H. 1950. Morphology of jaw muscles in birds. Zoological Annual Anatomy 70: 427-556. In German.
Huxley, T.H. 1867. On the classification of birds, and on the taxonomic value of the modifications of certainof the cranial bones observable in the class. Proc. Zool. Soc. London. pp. 415-472.
Jollie, M.T. 1957. The head skeleton of the chiken and remarks on the anatomy of this region in other birds. J.
Morph. 100(3): 389-436.
Jollie, M. 1977. A contribution to the morphology and phylogeny of the Falconiformes. Evolutionary
Theory 2: 101-345.
Ladygin, A. 1992. Kleptoparasitism among Steller’s Sea Eagle on Kamtchatka Peninnsula. Proceedings of the
Raptor Research Foundation Annual Meeting. November 11-15, 1992, Bellevue. p.117.
Ladygin, A. 1994. The hyoid apparatus in some Falconiformes. Zoological Journal 64(3): 63-76. In Russian.
Ladygin, A. 1994. Relationships of raptors wintering on the places of salmon spawning on the Kurilsky Lake
(South Kamtchatka). Current Ornithology 1: 96-106. In Russian.
Ladyguin, A. 1996. Foraging strategy of three raptors in salmon spawning ground in Kamtchatka Peninsula,
Russian Far East. Proceedings of the Second International Conference on Raptors, Urbino, Italy, 2-5October 1996.
Ladyguin, A. 1997. Group behaviour of Steller’s Sea Eagle on Kamtchatka. Zoological Journal 76(1): 83-93.
In Russian.
Lakjer, T. 1926. Studies of Jaw Muscles Innervated by Nervus Trigeminus in Sauropsidae. P. Hertzel,
Kopengagen. In German.
Marinelly, W. 1936. Skull and Visceral Sceleton of Birds -Guidance to the Comparison Anatomy ofVertebrates-. Von Bolk, Goeppert, Kallius and Lubosch. In German.
Pycraft, W.P. 1898. Contribution to the osteology of birds. Part II – Impennes. Proc. Zool. Soc. London 15:458-989.
Parker, T.J. 1890. Observations on the anatomy and development of Apteryx. Phil. Trans. Roy. Soc. London,
Ser. B. 182: 25-139.
Parker, W.K. 1873. On the structure and development of the bird’s. Transact. Linn. Soc., II ser. Zool. 1(3): 99154.
Sushkin, P.P. 1899. To the Morphology of Bird’s Skeleton -The Skull of Falco tinunculus-. Nouv. Memory
Soc. Imp. Natur, Moscow. In German with Russian summary.
Sushkin, P.P. 1902. To the Morphology of Bird’s Skeleton. Comparative Osteology of the Diurnal Birds ofPrey Accipiters and the Problems of Classification Part 1-2. Moscow University, Moscow.