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inexperienced and/or young individuals (Agostini 2005,
Agostini and Malara 1997, Agostini et al. 2000, 2004a,
2004b, 2005).
Whenaraptorfacesopenwater,severalfactorsinu-
ence its decision of whether or not to attempt a crossing:
the morphology, the distribution of landmarks and lead-
ing lines, the length of the water crossing, the physiologi-
calconditionofthebird,ockingbehaviour,timeofday,
and previous experiences; moreover, weather conditions
strongly affect water crossing behaviour (Kerlinger 1985,
1989, Agostini et al. 1994, 2002, 2003, 2005, Agostini
and Duchi 1994, Agostini and Panuccio 2003a, 2003b,
Klaas sen et al. 2010, Meyer et al. 2000, 2003, Panuccio
2005, 2011, Thorup et al. 2003). Thus, it is not surpris-
ing that at some sites, accurately counting migrating rap-
torsapproachingawatercrossingcanbedifcult;indeed,
in such cases Accipitriformes sometimes interrupt migra-
tion, disappear from view and pass again through the study
siteafterseveralhours.Raptorsockssometimeshesitate
and split up when faced with a water crossing and some-
timesyintheoppositedirectionofmigration(Bildstein
et al.2007).Becauseofthisbehaviourmanycoastalare-
INTRODUCTION
During migration Accipitriformes mostly use soaring
ight, thus optimizing the use of thermal currents, and
avoidinglongwatercrossingtolimitpoweredightover
water and to reduce energetic costs (Kerlinger 1989). The
wing morphology of raptors plays a role in the evolution of
migration strategies; in particular, species with relatively
broad wings (low aspect ratio) are less suited to undertake
the crossing of large bodies of water while the same, heav-
ier,speciesmaximizetheirightspeedoverland.Inpar-
ticular Kerlinger (1989) proposed the aspect ratio [(Wing
Span)2/(Wing Area)] as one of the most important mor-
phologic features explaining water crossing tendency of
raptors, while other authors highlighted the weight differ-
ences(SpaarandBruderer1997,ÅkessonandHedenström
2007, Pennycuik 2008). As a result, large soaring raptors
likevultures,eagles and common buzzardsareobserved
in small numbers during spring and autumn migration in
Central Mediterranean. Observations of broad-winged
raptorsinthis area donotgenerallyreectthe existence
ofatruemigrationyway,butrathertheymostlyconcern
A comparison of two methods for monitoring migrating
broad-winged Raptors approaching a long water crossing
Michele Panuccio¹,², Marco Gustin³, GiusePPe BoGliani¹
¹ Università di Pavia, Dipartimento di Scienze della Terra e dell’Ambiente - Via Ferrata 1, 27100 Pavia (Italy)
(panucciomichele@gmail.com)
² MEDRAPTORS (Mediterranean Raptor Migration Network) - Via Mario Fioretti 18, 00152 Rome (Italy)
³ LIPU (Lega Italiana Protezione Uccelli) - Via Trento 49, 43122 Parma (Italy)
Abstract–ObservationsonraptormigrationwerecarriedoutattheislandofMarettimo(CentralMediterranean)duringthersthalfof
October 2007. Raptors concentrate at this site before crossing the Channel of Sicily en route to Africa (130 km). We used and compared
two different census methods in order to limit the problem of double-counting of migrating raptors. A total of 510 and 299 individuals
wererespectivelycountedusingtwodifferentmethods:therstoneisthestandardmethodinusetocountmigratingraptors(all-occur-
rence), the second one consists in considering the maximum number of raptors observed together for each observation day (maximum-
daily-count). Raptors observed were almost juveniles and immatures (inexperienced individuals). For short-toed snake eagle Circaetus
gallicus, booted eagle Aquila pennataandcommonbuzzardButeo buteo,theresultsshowsignicantdifferencesincounts.Weathercon-
ditions affected observed numbers of these species at the site, although they did so similarly for both methods. Results show that, at Ma-
rettimo, it is not possible to make an accurate count of inexperienced broad-winged raptors approaching the long water crossing. For this
reason we suggest to consider as an activity index the daily count of the maximum number of birds of the same species observed together
duringeachdayofeldwork.
13
Avocetta 35: 13-17 (2011)
Panuccio et al.
as(likeCapBonPeninsula,MountConero,CirceoProm-
ontory), were previously considered unsuitable for count-
ing migrating raptors (Agostini 2005, Agostini et al. 1994,
Agostini and Duchi 1994, Agostini and Panuccio 2003a,
2003b, Panuccio 2005, Panuccio et al. 2004, Premuda et
al. 2008).
ThedistancebetweenAfricaandItalyisnarrowestat
the Channel of Sicily between the island of Marettimo and
theTunisianpeninsulaofCapBon;inthisareathousands
of migrating raptors are regularly observed during autumn
(Agostini et al.2000,2004b).Inparticular,betweenmid-
August and the end of September large numbers of black
kites Milvus migrans,honeybuzzardsPernis apivorus and
marsh harriers Circus aeruginosus are observed. Later in
the season, a smaller number of raptors, mostly juvenile
short-toed snake eagles Circaetus gallicus are recorded
(Agostini et al. 2004a, 2005).
The aim of this paper is to verify whether it is possible
to make an accurate count of large soaring raptors migrat-
ing through the island of Marettimo en route to Africa, and
to compare two different methods for doing so.
STUDY AREA AND METHODS
The island of Marettimo (Fig. 1) is located 30 Km off
WesternSicilyand130KmNEoftheCapBonPeninsula
(Tunisia). Observations were made from the 3rd to the 15th
October 2007 using a single watch point located along the
main ridge of the island at an elevation of 500 m above sea
level. Two observers with binoculars and telescopes were
employed. Observations were made from 9:00 (solar time)
until dusk.
Two different methods were used to count raptors.
Therstmethod isthestandard oneinobservationstud-
ies(Bibbyet al. 2000,Bildsteinet al. 2007) and hereaf-
ter called “all-occurrence”: it consists in counting raptors
when they passed over the watch point; in order to avoid
counting them again, birds were followed with binoculars
andtelescopeinordertoensureindividualsandocksre-
allywereleavingtheisland.Raptorsobservedyinginthe
opposite direction of the presumed optimal migration were
notconsideredin thecount(Bibby et al. 2000, Panuccio
2005).
The second method, hereafter called “maximum-daily-
count”, consists to count the maximum number of individ-
ualsofthesamespeciesobservedtogetheroneachday.In
both cases, we noted whether any individual birds could be
identiedbyplumagepeculiarities;moreover,birdsroost-
ing at the site were counted only if they were not observed
the following morning.
We tested the hypothesis that local weather conditions
affect the count of migrating raptors. We considered the
daily counts of the three species that show signicantly
different results with the two census methods: the short-
toed snake eagle, the booted eagle Aquila pennata and the
commonbuzzardButeo buteo.AGeneralizedLinearMod-
el (GLM) with negative binomial distribution was applied
using as independent factors: i) wind direction, ii) wind
speed (km/h), iii) temperature (°C), iv) air pressure (mbar)
and v) humidity (%) to explain variation in daily counts
of each species. These variables were obtained from the
meteorological station of the Trapani Airport every day at
13.20. Wind direction was divided into two categories ac-
cording to the most common winds in the study period:
northerly and southerly winds. The GLM was repeated
with the results of both census techniques. A Spearman
test was run to avoid collinearity between the 5 variables;
air pressure was excluded from the analysis since it corre-
lates with wind direction (Rho=-0.63; P<0.05). We veri-
edthestatisticalpowerofeachGLMbyapplyingaSha-
piro-Wilktestontheresidueofthemodel.Itwasnonsig-
nicantinallcases(P>0.05).
14
Figure 1.Study area(M=IslandofMarettimo,CB =Cap Bon
Pe ninsula, CP = Circeo Promontory, MC = Mount Conero).
150 km
N
MC
CP
CB
M
Sicily
Tunisia
A comparison of two methods for monitoring migrating broad-winged Raptors
15
RESULTS AND DISCUSSION
During the entire survey, we counted 510 raptors with the
“all-occurrence” method and 299 with the “maximum-dai-
ly count” method, for a total of 12 species (Tab. 1). For
comparisons involving more than 10 birds, the differences
incountsbetweenthetwomethods appear to be signi-
cantforthreespecies:theshorttoedsnakeeagle(χ²=22.2,
d.f.1,P<0.01),thecommonbuzzard(χ²=41.2,d.f.1,P<
0.01)andthebootedeagle(χ²=5.9, d.f. 1, P< 0.05). For
the marsh harrier, the black kite and the honey buzzard
thedifferencesarenotsignicant,but onlyfewindividu-
alsofeachwereobserved.Broad-wingedraptorshesitated
infrontof thewatercrossingand infourcasesocks of
shorttoed snake eagles and common buzzards were ob-
servedying east towards Sicily, while eagles wereob-
served undertaking the water crossing toward Africa in on-
ly three cases.
Forall these three species, weather conditions inu-
enced the number of individuals recorded, to a similar ex-
tent for both census techniques (Tab. 2). Wind speed in
the case of short-toed snake eagle and booted eagle and
winddirectioninthecaseofcommonbuzzardstronglyaf-
fectedobservednumberofmigrants.Inparticular,within-
creasing wind speeds numbers of short-toed snake eagles
and booted eagles decreased, while with southerly winds
(head-winds) lower numbers of common buzzards were
recorded. These results are expected, since raptors tend
to avoid water crossings when wind conditions require
an extra amount of energy consumption, as such as when
strong winds or headwinds are blowing (Meyer et al. 2000,
2003, Panuccio 2011, Panuccio et al.2002).Inthecaseof
short-toedsnakeeaglesand common buzzards, observed
numbers were also positively correlated with tempera-
tures; however, since temperature decreased as the days
passed (Rho=-0.72; P<0.01), we suggest that this corre-
lation could be related to the migration timing of the two
species.Resultssuggestthatweatherconditionsinuence
daily counts similarly for both methods probably because
raptors do not reach the island of Marettimo with unfa-
vourable wind conditions, as in these cases they do not un-
dertake the short water crossing between Sicily and Maret-
timo (approx. 30 km).
Previous studies which were carried out at the same
site showed that other species of Accipitriformes, such as
blackkites,honey buzzards, egyptianvulturesNeophron
percnopterus and marsh harriers were more likely to un-
dertake the water crossing en route to Africa (Agostini
et al. 2000, Agostini et al. 2004b, Ceccolini et al. 2009,
Panuccio 2005). Different behaviours also reect differ-
entmigrationstrategies:indeed,themigrationywaysof
those species pass over the Central Mediterranean area
and the island of Marettimo. On the other hand, short-toed
snakeeagles, bootedeaglesandcommonbuzzardsrarely
cross the central Mediterranean, preferring to use alterna-
tive yways or wintering mostly in Europe (Agostini et
al. 2000, 2004a, 2005, Agostini and Malara 1997, Isen-
mann et al. 2005). Additional support for the above hy-
pothesis comes from an analysis of the age classes of birds
observed in the present study, which were mostly young,
inexperienced individuals (Tab. 3).
This study shows that at the island of Marettimo it is
better do not use the same census method for all species of
migrating raptors. Since an accurate count of raptors mi-
Table 1. Numbers of raptors counted by two census methods.
SPECIES
Short-toed snake eagle Circaetus gallicus
BootedeagleAquila pennata
Lesser spotted eagle Aquila pomarina
Bonelli’seagleAquila fasciata
Egyptian vulture Neophron percnopterus
CommonbuzzardButeo buteo
HoneybuzzardPernis apivorus
BlackkiteMilvus migrans
Marsh harrier Circus aeruginosus
HenharrierCircus cyaneus
Pallid harrier Circus macrourus
Circus pygargus/macrourus
TOTAL
ALL-OCCURRENCE
210
44
10
1
1
144
11
33
51
2
1
2
510
MAXIMUM-DAILY-COUNT
123
23
6
1
1
53
10
32
44
2
1
2
299
Panuccio et al.
16
grating through Marettimo is unrealistic, we suggest that
broad-winged raptors should be “counted” by considering
the maximum number of individuals of the same species
observed at the same time on each day. Since recent stud-
iesusingsatellitetrackingshoweda highdegreeofex-
ibility between different individuals and years (Mellone et
al. 2011, Vardanis et al. 2011), we cannot exclude that the
proposedcensus techniquecouldalsobeawed andbias
the data. For this reason, this method should be seen more
as an “activity index rather than an actual estimate of the
numbers of individuals passing through”(Bildstein et al.
2007) the island of Marettimo, mainly considering that an
accurate count is not possible in the case of inexperienced
broad-winged raptors approaching a long water crossing.
Acknowledgements – This study has been made on behalf of the
LIPUBirdLifeItaly(LegaItalianaProtezioneUccelli).Wewould
liketothankAntoninoProvenzaandVitoVaccarofortheirhelp.
Table 2. Results of the GLM investigating the relationship between daily count numbers (dependent variable) and weather variables. The
asteriskindicatesthesignicanceoftheweathervariableinthemodel.
Short- toed snake eagle
Bootedeagle
Commonbuzzard
All occurrence
Maximum daily count
All occurrence
Maximum daily count
All occurrence
Maximum daily count
Explanatory Term
Wind direction
Wind speed
Temperature
Humidity
Wind direction
Wind speed
Temperature
Humidity
Wind direction
Wind speed
Temperature
Humidity
Wind direction
Wind speed
Temperature
Humidity
Wind direction
Wind speed
Temperature
Humidity
Wind direction
Wind speed
Temperature
Humidity
Estimate
1.0
-0.1
-0.7
-0.07
1.0
-0.1
-0.7
-0.07
1.2
-0.1
-0.3
-0.04
1.2
-0.1
-0.3
-0.04
1.8
-0.02
-0.9
-0.03
1.8
-0.03
-0.9
-0.03
Z value
1.41
-2.7
-3.0
-1.8
1.4
-2.8
-3.1
-1.8
1.9
-2.5
-1.2
-1.2
1.8
-2.5
-1.1
-1.2
2.4
-0.6
-3.6
-0.8
2.4
-0.6
-3.6
-0.8
P (> |z|)
>0.05
<0.01*
<0.01*
>0.05
>0.05
<0.01*
<0.01*
>0.05
>0.05
<0.05*
>0.05
>0.05
>0.05
<0.05*
>0.05
>0.05
<0.05*
>0.05
<0.01*
>0.05
<0.05*
>0.05
<0.01*
>0.05
S.E.
0.7
0.04
0.2
0.04
0.7
0.04
0.2
0.04
0.7
0.05
0.2
0.04
0.7
0.05
0.2
0.03
0.8
0.04
0.3
0.04
0.8
0.04
0.3
0.03
Table 3. Percentage of adults and juveniles/immatures observed at the site.
Species
Short-toed snake eagle
Bootedeagle
Lesser spotted eagle
Commonbuzzard
Honeybuzzard
Blackkite
Marsh harrier
Adult (%)
4.2
12.5
0
15.1
0
7.1
42
Juveniles/immatures (%)
95.8
87.5
100
84.9
100
92.9
58
Sample size
95
8
6
33
9
28
31
A comparison of two methods for monitoring migrating broad-winged Raptors
17
We would like to thank MEDRAPTORS (www.raptormigration.
org), a network of ornithologists and birdwatchers involved in the
research and the protection of migrating birds of prey through spe-
cicprojectsandobservationcamps.
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Associate Editor: G. Tellini Florenzano