Ornis Hungarica 2021. 29(1): 139–148.
DOI: 10.2478/orhu-2021-0011

The possible occurrence of cranial
asymmetry in three harrier (Accipitridae:
Circus) species
Tibor Pecsics1*, András Marx2 & Tibor Csörgő3
Received: May 21, 2021 – Revised: June 02, 2021 – Accepted: June 03, 2021
Pecsics, T., Marx, A. & Csörgő, T. 2021. The possible occurrence of cranial asymmetry inthree harrier (Accipitridae: Circus) species. – Ornis Hungarica 29(1): 139–148. DOI: 10.2478/
orhu-2021-0011
Abstract The harriers (Accipitridae: Circus) represent a unique group of raptorial birds due to their hunting
behaviour and their facial ruff and prominent facial disc. During previous studies it was suggested that harrier
species may have other convergent features shared with owls like asymmetric or enlarged ear openings related
to sensitive hearing capabilities. In this study, cranial asymmetry was done using SAGE (Symmetry andAsymmetry of Geometric Data) software. 32 skulls of 3 species (Western Marsh Harrier (Circus aeruginosus)
n=8, Montagu’s Harrier (Circus pygargus) n=10, Hen Harrier (Circus cyaneus) n=14) were photographed,digitized and assigned with 2D landmarks with TpsDig software. The variables were analysed based on
Generalized Procrustes analysis. The morphometric data showed cranial asymmetry of harriers. This asymmetry
should rather be explained by foraging strategies as the results are corresponding to the exceptionally goodhearing of these species among diurnal raptors.
Keywords: cranial morphology, morphometrics, anatomy, skull, shape, convergent evolution, harrier, auditory
systemÖsszefoglalás A rétihéják (Accipitridae: Circus) egyedi képviselői a ragadozó madaraknak, köszönhetően sajátos vadászati technikájuknak és megjelenésüknek, a tollak által alkotott gallérral és arcfátyollal. A korábbi kutatások során felvetődött, hogy utóbbiakon kívül más konvergens tulajdonságokon is osztozhatnak a baglyokkal, mint
amilyen az aszimmetrikus vagy megnagyobbodott fülnyílás, amely érzékeny hallásukra utal. Jelen tanulmányban
a koponya aszimmetria vizsgálatát a SAGE (Symmetry and Asymmetry of Geometric Data) szoftver segítségével végeztük. Összesen 3 faj 32 koponyájának (barna rétihéja – Circus aeruginosus, n=8, hamvas rétihéja – Circus pygargus, n=10, kékes rétihéja – Circus cyaneus, n=14) fényképét digitalizálva, kétdimenziós landmarkokkal
láttuk el azokat a TpsDig programban. A változókat Prokrusztész analízis segítségével vizsgáltuk, ahol a morfometriai adatok aszimmetriát mutattak a rétihéja koponyákon. Ez az aszimmetria a táplálkozásmóddal és viselkedéssel köthető össze és megerősíti azt a megállapítást, miszerint nappali ragadozókhoz mérten ezek a madarak kifinomult hallással rendelkeznek.
Kulcsszavak: koponyasajátosságok, morfometria, anatómia, koponya, alak, konvergens evolúció, rétihéja, táplálékpreferencia, auditív érzékelés
1
Behavioural Ecology Group, Department of Systematic Zoology and Ecology, Eötvös Loránd University, 1117
Budapest, Pázmány Péter sétány 1/C, Hungary
2
Department of Ethology, Eötvös Loránd University, 1117 Budapest, Pázmány Péter sétány 1/C, Hungary
3
Department of Anatomy, Cell- and Developmental Biology, Eötvös Loránd University, 1117 Budapest, Pázmány
Péter sétány 1/C, Hungary*corresponding author, e-mail: nobilis.equus@gmail.com

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ORNIS HUNGARICA 2021. 29(1)Introduction
The harriers (Accipitridae: Circus) represent a unique group of raptorial birds due to their
particular hunting behaviour (Redpath 1992), low, quartering flight, exceptionally lightwing loading, their mating pattern of polygyny. In their external appearance, harriers are
one of the most easily recognisable Accipitrinae raptors due to their external features suchas facial ruff and prominent facial disc (Simmons 2000). Harriers have traditionally beenplaced within the subfamily Circinae (Peters 1931), but recent molecular studies haverevealed that harriers are more closely related to Accipiter (Nagy & Tökölyi 2014, Oatley
et al. 2015). The feeding strategies are highly diverse and the morphological adaptations
for feeding are a notable feature of avian evolution (Zusi 1993, Zweers et al. 1994). As inmany other avian groups, food supply is one of the main factors influencing the ecologyof raptors. Population densities of raptors are often limited by food supply (Newton 1980,
Franklin et al. 2000), which also influences the number of specialist hunters being more
likely to fluctuate annually when dependent on a fluctuating food source (Hamerstrom 1969,
Korpimäki & Norrdahl 1991, Butet & Leroux 1993).
Previously, the adaptive significance of the anatomical and behavioural convergence
between the harriers and some owl species was examined (Clark et al. 2020). Anatomically,both groups evolved a sound collecting facial ruff, a curved structure of skin and feathers
surrounding the ears, and forage very close to the ground, harriers are very vocal birds while
gliding over the hunting area (Rice 1980). The harriers circle an area several times listening
and looking for prey, as they have exceptionally good hearing among diurnal raptors. It
suggests that these species have a highly developed auditory system. The ears are coveredby the feathers of the facial disc (Rice 1982). Some owl species also have a pronounced
facial disc, guiding sounds into the ear openings. In owls, the beak is pointed downward,
increasing the surface area over which the soundwaves are collected by the facial disc
(Nishikawa 2002). Many owl species are remarkable for the bilateral asymmetry of the ears
which attributes support a highly developed sense of directional hearing (Coles & Guppy
1988, Pecsics et al. 2018). However, the differences are represented by different tissuesof the head (feathers, earflaps, ear holes, etc.), ear asymmetry basically caused by cranial
structures only, due to the different positions and orientations of the squamoso-occipitalwings. Ear morphology is different in structure and geometry between species (Norberg
1978). The symmetrical ears are representing a most basal form and cranial structure(Nishikawa 2002).
Bilateral symmetry is a subject of widespread interest, and structures with such symmetry
are particularly considered when these consist of two mirror copies on opposite sides ofthe body (Klingenberg et al. 2002). Bilateral symmetry analysis focuses on identifying andmeasuring the location and extent of symmetry departures in structures, like in the context
of matching symmetry, where the two mirror images are considered separated parts of thestructure (Torcida et al. 2016).
Ear adaptations are a well-known feature of owl evolution. Also there are some documented
size and shape asymmetries in birds (Norbeg 1978, Aparicio & Bonal 2002, Güntürkün et al.
2000, Parés-Casanova & Salas-Bosch 2020). It was suggested that harrier species may haveT. Pecsics, A. Marx & T. Csörgő

141

asymmetric or enlarged ear openings for accurately pinpointing sound (Van Grouw 2012),
but our knowledge is still limited regarding the degree and the direction of these skeletal
asymmetries.
In this preliminary study, we investigated the cranial diversity among three harrier species.
Our objective was to increase our knowledge in the relationship between skull shape and thepossible asymmetric skeletal features. For this reason, we investigated shape asymmetries
related to hearing capabilities.

Materials and Methods
Specimens
This study is based on 32 skulls of 3 species: Western Marsh Harrier (Circus aerugi­
nosus) n=8, Montagu’s Harrier (Circus
pygargus) n=10, Hen Harrier (Circus cyaneus) n=14. All skulls are from adult specimens belonging to the collection of Eötvös
Loránd University (Budapest, Hungary), thecollection of the Hungarian Natural History
Museum (Budapest, Hungary), Mátra Museum (Gyöngyös, Hungary) and the digital archives of Natural Sciences Museum of Barcelona (Barcelona, Spain). No bird has been
killed to obtain its skull; all birds died eitherof natural causes, whether accidental deathor whether death in captivity.
Landmarks and procedures
The variation of cranial morphology was
analysed using landmark-based geometricmorphometry. In this study, we tried tofind landmarks for this analysis to cover
the geometric form of the regio oticalis.
Ideal landmarks are discrete and noticeable
anatomical features that do not alter their
topological positions, providing adequate
coverage of the morphology (Zelditch etal. 2004). The landmarks were taken fromhigh resolution (1200×1600 pixels) photos.
We took 3 photographs from each specimen

Figure 1. Position and number of landmarks. A:
fixed landmarks in caudal view, B: fixed
landmarks in lateral (left-right) view
(numbers correspond to Table 1)
1. ábra A vizsgálatban használt landmarkok
száma és pozíciója. A: fix landmarkok
hátulnézetből, B: fix landmarkok
oldalnézetből (bal-jobb) (a számokmegnevezését lásd a 1. táblázatban)

ORNIS HUNGARICA 2021. 29(1)142(caudal, lateral – right and left). Images were standardised for the foramen magnum occipitale
and the tip of the mandible. Testing for fluctuating asymmetry, three measurements weretaken (replicates) per individual. For each specimen, 22 fixed landmarks were recordedin caudal view and 25 fixed landmarks in lateral view (right and left) (Figure 1) (Table 1).
The coordinates of the landmarks were digitised using TpsDig 2.16 software (Rohlf 2010)
and were transformed using the Procrustes superimposition method. We used MannWhitney U two-tailed test for comparing the sides. To detect the components of variances
Table 1.

Number and description of landmarks in caudal (A) and lateral (B) view (terminology
according to Baumel 1993, Sun et al. 2018)
1. táblázat Az egyes landmarkok száma és leírása, hátulnézetben (A) és oldalnézetben (B) (terminológia Baumel 1993 és Sun et al. 2018 alapján)A) Number oflandmark1

mid section of the neurocranium

2 , 22highest point of the neurocranium

3 , 21

top of curvature at the superior end of the M. add. mandibular externus scar

4 , 20

top of curvature at the caudal end of the M. add. mandibular externus scar

5 , 19

top of curvature at the inferior end of the M. add. mandibular externus scar

6 , 18highest edge of the temporal wing

7 , 17

top of curvature at the superior end of the temporal wing

8 , 16

top of curvature at the caudal end of the temporal wing

Description of landmark

9 , 15

top of curvature at inferior end of the temporal wing

10 , 14

lowest edge of the temporal wing

11 , 13most inner edge of the temporal wing at the inferior end

12B) Number oflandmark1lowest point of condylus occipitalis
Description of landmark
prominentia cerebellaris

2 , 14highest point of the neurocranium

3 , 15

mid-point of the cranio-facial hinge

4 , 16

top of curvature at the caudal end of the external nares

5 , 17

top of curvature at the rostral end of the external nares

6 , 18tip of the beak

7 , 19articulation point of jugal bar and maxilla

8 , 20the processus of the opisthotic

9 , 21

lowest edge of the temporal wing

10 , 22

top of curvature at the inferior end of the temporal wing

11 , 23

top of curvature at the superior end of the temporal wing

12 , 24highest edge of the temporal wing

13 , 25

processus postorbitalis

T. Pecsics, A. Marx & T. Csörgő

143

and deviations, Procrustes ANOVA was used (Klingenberg et al. 1998, Marquez 2006).
Consensus configurations and relative warps were conducted. Variability in shape wasassessed using the scores obtained for each individual on the ﬁrst two relative warps.
Cranial asymmetry was estimated using SAGE (Symmetry and Asymmetry of GeometricData) software (Marquez 2006). We conducted principal component analyses (PCA) on
these morphological variables in caudal view. The relative warps are corresponding to theprincipal components (PCs) and define the shape space in which individuals are replaced.
We used PAST v.1.7 software (Hammer et al. 2001) for principal component analysis andto extract deformation grids. We only considered those PCs which are explaining >10% of
the variance.

Results
Mann-Whitney U two-tailed test showed significant differences between the two sides of theskull regarding the temporal region (U=838.5; z=-8.22, P<0.001 at the significance level 0.01).
Procrustes ANOVAs revealed that directional asymmetry was greater than fluctuating
asymmetry (Table 2) reflecting the coordinates in caudal and lateral view.
During the second analysis, we used 22 fixed landmarks recorded in caudal view. The
first two PCs explained 38%, and 22% of the variance in shape. The first PC described thevariation in shape, the second the relative orientation of the temporal wing which showedthat Circus species differ considerably in their degree of asymmetry (Figure 2). The Western
Marsh Harrier showed less cranial asymmetry than the other two species, Montagu’s Harrier
was in intermediate position in the analysis and Hen Harrier showed the most asymmetricscores (Figure 3).
Table 2.

Results of ANOVA for the registered coordinates in caudal (above) and lateral view
(bottom), with the effect of “side” (directional asymmetry) and “side*individual” effect(fluctuating asymmetry)
2. táblázat Az ANOVA eredményei a regisztrált koordináták tekintetében hátulnézetben (fent) ésoldalnézetben (alul), az “oldal” (direkcionális aszimmetria) és az “oldal*egyed” (fluktuáló
aszimmetria) hatásával
SS

MS

df

F

p

individual / egyed

effect / hatás

0.05660.0004160

4.69480.001

side / oldal

0.00450.000220

2.96940.001

16.7492

0.001

side*individual / oldal*egyed

0.01210.0001160

error / hiba

0.00320.0002

720

SS

MS

df

F

p

0.06460.0004184

4.78440.0001effect / hatás
individual / egyedside / oldal

0.00730.000323

4.346

0.0001side*individual / oldal*egyed

0.01350.0001

184

93.48480.001

error / hiba

0.00060.0001

828

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ORNIS HUNGARICA 2021. 29(1)Figure 2. Various harrier species differ considerably in their degree of asymmetry (in caudal view). A:
Western Marsh Harrier (Circus aeruginosus), B: Montagu’s Harrier (Circus pygargus), C: Hen
Harrier (Circus cyaneus)
2. ábra Az egyes rétihéja fajok különböznek az aszimmetria mértékében (hátulnézetben. A: barnarétihéja (Circus aeruginosus), B: hamvas rétihéja (Circus pygargus), C: kékes rétihéja (Circus
cyaneus)Figure 3. Differences in position and shape of left and right tympanic wings on the skull of a Hen
Harrier (Circus cyaneus) specimen
3. ábra A kékes rétihéja (Circus cyaneus) egy példányának koponyáján lévő különbségek a bal és
jobb oldali halántékcsont tympanicus nyúlványainak helyzetében és alakjában

T. Pecsics, A. Marx & T. Csörgő

145

Discussion
We found that the influence of directional asymmetry was greater than fluctuating asymmetry
which suggests that these asymmetrical features probably evolved to support directional
hearing and sound localization. In lateral and caudal view, there are differences in thesize and orientation of the temporal wing between the species. However, the anatomicalsimilarities between harriers can be explained by their slightly different foraging methods
and hunting behaviour.
Comparisons between sympatric raptor species with similar ecological needs andpreferences but different body size and social behaviour are helpful in assessing the influenceof energy needs or dominance on foraging strategies and food partitioning (Fedriani etal. 2000, Buij et al. 2011). The Montagu’s Harrier and the Hen Harrier are breeding andforaging sympatrically in many areas. The diet of these species is very similar (Millon et al.
2002). The species differ in size, Hen Harrier being larger and also there are differences intheir breeding system (Cramp 1980). Food partitioning by size between species that differ
in food requirements is expected and has been observed in other species too (Gerstell &Bednarz 1999). Accordingly, within the large spectrum of prey types indicated that Hen
Harriers preyed more frequently on larger prey and Montagu’s Harriers on smaller prey(including more insects). This kind of segregation was observed in the main prey (small
lagomorphs and rodents), and in the sizes of the most important alternative prey (Garcia &Arroyo 2002, Arroyo & Garcia 2006, Arroyo 2008).
The competition between Western Marsh and Hen Harriers for hunting space in the same
area was also observed, i.e. Western Marsh Harriers hunted mainly over the marsh (Buij2012, Tornberg & Haapala 2013) resulting in niche separation between the two species.
Western Marsh Harrier has proportionately shorter wings and tail, its flight is slower andless manoeuvrable compared to Hen Harrier. Western Marsh Harrier has longer tarsi whichshould be an adaptation to reach deep into tall marsh vegetation preferred to hunt, maybe
because this gave it more opportunity for surprise during hunting (Clarke et al. 1993,
Cardador et al. 2012).
Differences in foraging efficiency between generalist and specialist species could be
explained by neural limitations. This means that generalists need learning to be able tonarrow their resource choices (Bernays et al. 2004). Specialization may be also associatedwith morphological or behavioural adaptations to handle certain resources. Harriers are
diurnal raptors but evolved a parabolic collar of feathers surrounding the eyes, suggestingthat hearing is enhanced to locate small mammal prey hiding in dense vegetation (Rice
1982, Redpath 1992, Simmons 2000). The facial disc is more pronounced in those harrier
species which are feeding on mainly small rodent prey like the Pallid Harrier (Circus
macrourus) (Buij 2012). The facial disc encircling the ears and meeting below the beak,in contrast, the facial disc of the Montagu’s Harrier is confined to an arc just behind theear openings, suggesting its hearing capacities might be less well developed than thatof the rodent specialist species (Clarke et al. 2008, Terraube et al. 2011). This is also inaccordance with our results. The Western Marsh Harrier is a great opportunist with lackof specialization (which means very wide range of prey, varying with local availability)

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ORNIS HUNGARICA 2021. 29(1)showed less asymmetrical features. The Montagu’s Harrier is feeding on small ground birds,
small mammals, reptiles and large insects (Thiollay 1994) but the Hen Harrier (which preys
on mostly small rodents, like voles, mice, cotton rats, ground squirrels) (Garcia & Arroyo2004) showed greater asymmetries in temporal region than the other two species. It suggeststhat feeding mainly on small rodents requires more sensitive auditory capabilities to detect
the movement of the prey item. It is also possible that the relative size of the facial discis in relationship with asymmetrical cranial structures, which also connected with hearing
and auditory capabilities in owls (Norberg 2002). In the case of Hen Harrier some kind of
nocturnal behaviour (Russell 1991) may have also effect on this feature.
Our results show relationships between cranial asymmetry and hunting behaviour but also
highlight the morphological complexity and diversity of raptor skulls. Future studies should
rather examine the possible interspecific and intersexual differences that may have effect on
the cranial morphology and asymmetry of these birds. With a larger sample size (including
more harrier species) and using parameters of soft tissues of the ear and temporal region will
create a more complex vision regarding the evolution of asymmetrical features which aresupporting directional hearing. New investigations reflecting on cranial asymmetries may
increase our knowledge to understand how these special features are functioning in living
specimens.

Acknowledgements
We are grateful to Gergely Babocsay, Miklós Laczi, Tamás Kondor, Martin Segesdi and
Péter Urtz for the technical assistance. The text was supervised by Bridgette Dennett, Emese
Bodor and Jenő Nagy.
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