j. Field Ornithol., 60(3):369-379
JUVENILE DISPERSAL OF SPANISH IMPERIAL
EAGLES
LuIs M. GONZALEZ• AND BORJAHEREDIA1
¾CONA
Serwcio de Vida Silvestre
Gran Via San Francisco35 Madrid 28005, Spain
Josv•L. GONZALEZ
2 ANDJUAN C. ALONSO
2
2Museo Nacwnal de CienczasNaturales, CSIC
J. GutidrrezAbascal2, Madrid 28006, Spain
Abstract.--Postfiedging
dispersalofjuvenileSpanishImperial Eagles(Aquilaadalberti)was
studiedfrom 51 recordsof recoveries,sightings,and contactswith banded, and wing or
radio-taggedbirds. When they becomeindependentyoung eaglesleave their natal areas,
travellingup to 350 km. Dispersalcan be dividedinto three phases(1) exploratoryflights
with return to the natal area, (2) longdistancetrips away from the natal area during which
somebirds establishtemporary territories in areas of apparent food abundance,and (3)
return to the vicinityof the natal area. During dispersalthe youngsufferedhigh mortality,
which is attributed to the risks of dispersion"per se" and the lack of familiarity with the
new areasthey explore.The distancebetweennatal areasand recoveriesfirst increasedand
then decreasedwith age, the birds tending to return to the areas where they were born, a
fact probablyrelated to marked philopatty in this species.However, there is evidenceof at
least one individual breeding in an area different from its place of birth, which suggests
that there is somegeneticexchangeamongthe different sub-populations.
DISPERSION
DE LOSJOVENESDE AQUILAADALBERTI
Resumen.--En basea 51 registrosde avistamientos,contactosde individuosanilladoso con
radiotransmisoresy recaptur6 de aves se estudiasel patr6n de dispersi6nde j6venes de
Aquila adalberti.Cuando los j6venesse independizan,dejan sus fireas natales y viajan
distanciashastade 350 km. E1 patron de dispersi6nsepuededividir en tres fases.J6venes:
(1) ruelosexploratorios
conregresoa su fireanatal, (2) largosruelosfuera del fireanatal
en dondelas avesestablecenterritorios temporerosen lugarescon aparente abundanciade
alimentoy (3) regresoa la vecindaddel fireanatal. Durante la dispersi6nlosj6venessufren
una alta mortalidad,atribuidaa losriesgos
de la dispersi6n
per sey a la faltade familiaridad
conlas fireasque exploran.Las distanciasentre las fireasnatalesy lugaresde recuperaci6n
dej6venes,aumentay luegodisminuyeconla edadde las aves.Hay una tendenciaa regresar
a loslugaresnatales,probablemente
debidoa la marcadafilopatrla en la especie.Sin embargo,
hay evidenciade al menosun individuo, que se reprodujoen un firea diferente a la que
naci6,lo quc sugiercun cicrtointercambiogen•ticoentre diferentessub-poblaciones.
The mostfrequentdefinitionof dispersalis "permanentmovementan
individual makesfrom its birth site to the place where it reproducesor
would have reproducedif it had survivedand found a mate" (Howard
1960). The definitionrefersexplicitlyto the movementof prereproductive
individuals.Greenwoodand Harvey (1982) suggested
that dispersalfrom
the site or group of birth to that of first reproductionor potential reproductionshouldbe labellednatal dispersal.Subsequentmovementbetween
sitesor groupsshouldbe labelledbreedingdispersal.Juvenile dispersal
refersonly to the movements
of individualsoncethey becomeindependent
from their parents.
In most birds of prey, juveniles are more likely to perform longer
dispersalmovementsthan adults (Brown and Amadon 1968, Newton
369
370]
L. M. Gonzalez
etal.
J.Field
Ornithol.
Summer 1989
1979). Reasonsforjuveniledispersalinclude:ageof the individual,limited
food availability in the natal area, previousor geneticexperience,intrasexualcompetition,and territorial disputes(Adamcikand Keith 1978,
Houston 1978, Newton and Marquiss 1983, Snow 1968, and Stewart
1977). Many of these individualsreturn to their natal area to breed
(Newton 1979, Newton and Marquiss 1983).
Adult SpanishImperial Eagles(Aquilaadalberti)remainontheir breeding territories throughoutthe year (Gonzfilez 1989, Valverde 1960),
whereasjuvenilesleavetheir natal areaoncethe post-fledging
periodhas
ended (Alonso et al. 1987, Gonzfilez et al. 1985). The characteristicsof
their movements
at this stageare unknown(Bernis1966,Valverde1960).
In this paperwe describethesejuveniledispersalmovements
and discuss
them in relationto the ageof the individuals.
METHODS
Data are from: recoveriesof bandedindividualsfound dead,sightings
of wing-taggedindividuals,and radio contactsof ten youngeaglesradiotaggedin 1984 (Alonso et al. 1987, Gonzfilez et al. 1985). The data
represent51 band recoveriesand contactswhere placeof birth, age, and
locality of observationor banding recoverywere known.
From 1984 to 1986, 46 youngSpanishImperial Eaglesin southwestern
Spain were fitted with plasticwing color-markers.Markers were fitted
around the leading edgeof the humerusand securedat the pointedends
betweenthe tertiaries and scapularswith a pop rivet (Kochert 1972).
Tags were 750 x 350 mm. and weighed8 g. They were madeof Saflag
(SafetyFlag Co. of America, Pawtucket,Rhode Island).
Eagleswere color-markedindividuallyeachyear. Each was fittedwith
two coloredmarkers; one indicatedthe place of origin (Guadarrama,
Extremaduraor Dofiana, in central,westernand southof Spain respectively) and the other the year in which the individual was marked (1984,
1985, or 1986). Both tags were providedwith individual symbolsindicating the exact nest.
Ten youngwere marked individuallywith radio-transmittersat Dofia-
na National Park. The telemetryequipmentconsistedof solar-powered
transmitters,LA 12 DS receivers,and three-elementhand-held¾agiantennas, all from AVM-Instruments
Co., Livermore, California. The
transmitterpackagesweighed75-80 g, approximately2-2.5% of the body
weight of the bird, and had an estimatedaveragelifetime of 3 y. The
packageswere attachedto the eagleswith a back-packharness.No abnormal behaviorassociated
with the harnesses
or transmitterpackages
was observed.Receptionrangeswere about 5-8 km when the bird was
perchedon a tree, and 20-30 km when the bird was flying. Once the
youngstartedto fly long distances,they were trackedfrom vehiclesand
a Dornier
aircraft.
All marked and unmarkedindividualsreceivedaluminum leg-bands.
Chickswere taggedat approximately75% their normal fledgingage to
allow sufficientsizeand featherdevelopmentfor adequatemarker reten-
Vol.60,No.3
number
Dispersal
ofSpanish
Imperial
Eagles
[371
of
sightings
14.
lO
5
o
4-5
months
10-11
20-21
30-31
36-37
FIGURE1. Band recoveriesand wing-radio marker sightingsof young SpanishImperial
Eaglesin relation to age.
tion. We publishedannouncements
in technicalpublicationsand sent
circularsto universities,ornithologicalsocietiesand environmentalagencies,requestinginformation on sightingsof color-markedbirds. Not all
observations
of markedbirdsgaveus completeinformation;in thosecases
where nest of origin could not be identified, we consideredthe centroid
of all nestsmarkedthat yearwithin the placeof origin,followingSteenhof
et al. (1984).
Observationsand radio-contacts
of individualswhich had not yet completedthe post-fledgingperiod (age 4 mo, approximately)were not considered.we alsodid not considertwo chicksmarked in 1984, whosepostfledgingperiod and dispersalhad been abnormal due to the accidental
deathof their parents(Gonzfilezet al. 1986).
The age of young individuals was determinedto the nearestmonth.
As the majority of the birds hatchedin late April or the beginningof
May, we assigned"age one" to June 1st.
RESULTS
Tag lossobservedbetweentaggingand dispersalfrom the natal area
was 4% (r• = 46), similar to that observedin Golden Eagles (Aquila
chrysaetos)
(Kochertet al. 1983).There is a regulardecrease
in thenumber
of sightingsand recoverieswith time (Fig. 1).
We obtainedinformationfrom 9 banding recoveries,29 sightingsof
wing-markers and 13 radio-contacts(Table 1). To evaluate the effectivenessof wing-marking, we calculated wing-marked sighting rates
(numberof sightings/numberof marked birds). The rate was 43.4% (r•
= 46), similar to the oneobtainedwith GoldenEagles(33.2%, r• = 256)
(Kochertet al. 1983) andmuchlargerthanthat of Bald Eagles(Haliaeetus
leucocephalus)
(4.4%, rz = 56) (Gerrard et al. 1978). Sightingratesfor
SpanishImperial Eaglesfar exceedband recoveryrates (5.5%, r• = 109)
372]
L. M. Gonzalez
etal.
J.Field
Ornithol.
Summer 1989
(Fernfindez-Cruzand Asensio1982) showingthe high profitability of
wing-markingin relationto bandingin eaglepost-fledging
dispersalstudies.
Oncetheyoungcompleted
thepostfledging
periodin September(Alonso
et al. 1987, Gonzfilezet al. 1985) they left the natal area. Two of the
radio-markedindividualsundertooklong distanceflights,while the other
six made exploratoryflightsof considerable
length,sometimesreturning
to the natal area within the sameday (Table 1). These pre-dispersal
movementslasted 1-7 d, extended 20-135 km, and took different direc-
tions.Two individualstaggedin Dofiana were observedin the Gibraltar
area. In all theseflights,the youngwent alone and did not dependon
their parents.Final dispersionfrom the area took placebetween1 Sep.
and 3 Oct., at agesof 123-156 d (Table 1). The birdssetout in different
directions:one went NE, two E, and two SE. It was not possibleto
determinethe directionfollowedby the otherthree young.
From our data, we couldnot determinethat the youngbirds crossto
Africa over the Gibraltar Straits, even though individuals have been
observedthere (Table 1). However, the crossingof the Straits doesseem
to happen.Between1967 and 1974, at leastfour individualsdid follow
this route(Bensusan,in litt.) and othersimilarcaseshavebeenpublished
(Bernis 1980, Corteset al. 1980, Evans and Lathbury 1973, Pineau and
Giraud-Audine 1978). Furthermore, a bird banded in Dofiana in 1982
was recovereddead in Aim Jorra (Morocco) in 1984 (Bergier 1987;
Calder6n et al. 1988).
Accordingto the distancebetweenthe placeof birth and the placeof
recoveryor sighting,the juvenile dispersalcouldbe dividedin to three
differentphases(Fig. 2, seealsoTable 1). The first phaselastsapproximately3 mo (4-6 mo of age) and is characterized
by shortmovements
(• = 45.8; km; n = 18; SD = 45.1), and exploratoryflightsof the young
returningusuallyto the natal area. This phaseis coincidentwith the predispersivephasethat has been describedin other bird studies(Baker
1978, Lawn 1984).
The secondphaselasts approximately9 mo (7-15 mo of age) and
includes
juveniledispersal.
The youngtravellongerdistances
(• = 162.4
km; n = 16; SD = 121.0) than in the first phase,the differencebeing
significant(F•5,•7 = 7.1; P < 0.01). However,someindividualsstay in
thevicinityof thenatalareaandthusa gradationin thedispersalbehavior
of this species
appearsto exist,betweenthe longdistanceflightsand the
shorttrips within the natal area.
The third and last phasetakesplaceafter the 16th mo of age and is
markedby the existence
of recoveries
near the natal area (• = 41.9 km;
n = 15; SD = 64.9). The meandistanceis significantlysmaller(F•5,•4=
3.47; P < 0.01) than that of the secondphase.During this third phase,
thebirdsthat haveundergonedispersalandtravelledlongdistances
return
to the natal area.
The individualsmarked in Dofiana performedthe longesttrips. Dis-
persaldoesnot followa preferreddirection(G = 2.3; P > 0.05, Table
Vol.60,No.3
Dispersal
ofSpanish
Imperial
Eagles
[373
TABLE1. Band recoveries,
wing marker sightings,and radio trackingsightingsof young
SpanishImperial Eagleswith referenceof distancein km, orientationand age in years
(a) and months(m), from hatchingsiteto recoverysite.Symbolsare (EURING code):
1--ringed as pull.; '{'--shotor killed by man; •:--killed by electrocution;
RT--radio
trackingsighting;A--wing-mark sightings.
Compass
direction
Type
of
return
Dates
Origin and location
1
10.06.77
Toledo
:1:
00.04.78
Cenicientos,Madrid (Fernandez-Cruz,
1982)
1
13.06.80
Madrid
•'
26.10.81
Villanueva de los Infantes, Ciudad Real
1
22.05.76
Toledo
:1:
00.06.83 Malpartida de Plasencia,Caceres
1
.06.80
']'
23.05.83
1
.05.87
:1:
1
•:
1
16.10.87
.05.86
from
Disnatal
tance site
Age
(yr: mo)
70
NE
230
SSE
la.2m.
WSW
6a.llm.
--
3a.lm.
80
10m.
Toledo
Ventas con Pefia Aguilera, Toledo
2
Madrid
Villamantilla, Madrid
Caceres
ESE
6m.
105
20
NE
la.10m.
130
ENE
7m.
35
115
135
40
SE
SE
SE
SE
5m.
5m.
5m.
6m.
24.04.88 Torrejon el Rubio, Caceres
.05.85
Caceres
:1:
25.11.85 Quismondo,Toledo
1
RT
A
RT
.05.84
15.09.84
22.09.84
24.09.84
Dofiana, Huelva
Sanlucar, Cadiz
Tarifa, Cadiz
Tarifa, Cadiz
']'
25.10.84
Puerto Real, Cadiz
1
RT
.05.84
01.09.84
RT
1
RT
02.09.84
.05.84
21.09.84
Dofiana, Huelva
Dofiana, Huelva
Villamanrique de la Condesa,Sevilla
Doffann, Huelva
20
25
SSW
E
5m.
5m.
Villamanrique de la Condesa,Sevilla
30
NE
5m.
40
130
40
80
NE
SE
NE
5m.
5m.
5m.
9m.
80
80
SE
SE
11m.
12m.
120
E
12m.
130
SE
5m.
12
SE
6m.
E
8m.
1
.05.84
Dofiana, Huelva
RT
A
RT
RT
19.09.84 Villamanrique de la Condesa,Sevilla
21.09.84 Tarira, Cadiz
25.09.84 Villamanrique de la Condesa,Sevilla
06.02.85 Alcala de los Gazules, Cadiz
RT
RT
27.03.85
16.04.85
Alcala de los Gazules
Alcala de los Gazules
1
RT
1
RT
1
.05.84
16.04.85
.05.84
24.09.84
.05.84
Dofiana, Huelva
Serenil, Malaga
Doffann, Huelva
Tarira, Cadiz
Doffann, Huelva
RT
10.11.84
Doffann
1
A
.05.84
15.12.84
SE
Doffann, Huelva
Doffann
5
374]
L. M.
Gonzalez
et al.
J. Field Ornithol.
Summer
T^BLE
1.
1989
Continued.
Compass
direction
Dis-
from
natal
tance
site
Type
of
return
Dates
.05.84
Origin and location
Dofiana, Huelva
2
03.04.87
Dofiana
.05.84
00.03.86
Dofiana, Huelva
Cazalla, Sevilla
Dofiana, Huelva
.05.84
21.08.85
Dofiana
.05.84
26.01.86
Dofiana, Huelva
.05.84
Dofiana, Huelva
03.04.87
00.12.87
.05.85
00.12.85
00.01.86
00.02.86
Dofiana
28.08.85
23.08.86
.05.85
27.03.86
.05.86
20.08.86
.05.86
20.08.86
.05.86
20.10.86
.05.86
20.08.86
.05.86
00.02.87
.05.86
la. llm.
4
SSW
la.4m.
12
WSW
la.9m.
--
1
--
2a. llm.
3a.7m.
Dofiana, Huelva
Villamanrique, Ciudad Real
Viso del Marques, Ciudad Real
Villamanrique
Dofiana
Dofiana
.05.85
NNE
1
.05.85
07.03.86
.05.85
00.12.87
110
Dofiana
Amedina, Ciudad Real
12.11.86
23.11.86
00.01.87
00.07.87
2a.1 lm.
Dofiana
20.04.86
29.04.86
.05.85
Age
(yr: mo)
Villamanrique
8m.
9m.
NE
NE
NE
NE
NE
10m.
4
N
10m.
3
NE
NE
NE
SSE
325
305
325
340
325
12m.
12m.
Dofiana, Huelva
Dofiana
Dofiana
3
10
Dofiana
Vejer de la Frontera, Cadiz
110
la.6m.
la.7m.
la.9m.
2a.3m.
Dofiana, Huelva
1
Dofiana
--
2a.7m.
Madrid
E1 Tiemblo, Avila
25
w
4m.
Navas del Marques, Avila
35
NNW
la.4m.
Madrid
Alia, Caceres
Badajoz
Villar del Rey
Badajoz
Villar del Rey
Badajoz
Villar del Rey
Caceres
Aldea del Cano
125
SW
11m.
8
NNE
4m.
8
NWE
4m.
1
--
6m.
12
N
20
SW
10m.
95
SSW
SSW
8m.
15m.
4m.
Caceres
San Vicente
de Alcantara
06.01.87
Badajoz
Cheles, Badajoz
11.08.87
Cheles
115
Vol.
60,No.3
Kms
Dispersal
ofSpanish
Imperial
Eagles
340_
[375
ß
ß
300
ß
ß
ß
25O
200
150
I
ß
ßß
ß
100
ß
ß
ßß
ß
i
5
10
15
........
20
i
25
30
,
,
,
....
35
i
40
,
,
.
,
45
months
FIGURE2. Variation of the recoverydistancein kms in relation to age,in monthsof young
SpanishImperial Eagles,ringedas nestlings.Blackdotsrepresentsightingsand recoveries.
1). After dispersalhas taken place,someindividualssettledown in temporary territories. This behaviorwas observedin three young (Fig. 3):
the first had been radio-taggedin Dofiana in 1984, and was found 80
km away from the natal area from Februaryto April 1985. The second
waswing-taggedin Extremadurain 1986 andwasobserved
fromJanuary
to August1987 in an area alongthe Spanish-Portuguese
border,95-115
km from its place of birth. The third individual was wing-taggedin
Dofiana in 1985 and regularly seenin the plainsof La Mancha (Ciudad
Real) from December1985 to April 1986, the distancefrom the natal
area was 350 km. The existenceof temporaryterritorieshas beenmentionedfor otherbirdsof prey (Beske1982, Gargett 1975, Gerrard et al.
1978, Newton 1979, Picozziand Weir 1976), with the suggestion
that
this behaviorcan help birdsbecomefamiliar with new areaswhere they
couldbreedsubsequently
(Baker 1978).
DISCUSSION
The patternofjuveniledispersalin SpanishImperial Eaglesis similar
to the patterndescribed
for othereaglesof similarsize,suchasthe Golden
Eagle (Aquilachrysaetos)
and Wedge-tailedEagle (A. audax) (Ridpath
and Brooker 1986, Steenhof et al. 1984). We think that the decreasein
376]
L. M. Gonzalez
etal.
J.Field
Ornithol.
Summer
1989
France
Portugal
ß
ß
ß
?'
Atlantic
:diterraneanSea
Ocean
Gibraltar
Strait
IOOkm
,
•
,
(•
FIGURE3. Sightingsand recoverylocationsof youngSpanishImperial Eaglesthat moved
>20 km, from the marking location.Areas into open lines have beenusedto mark the
locationof the temporaryterritoriesestablished
by someyoung,and in continuousline
to mark the locationof the marking study areas.
the numberof bandrecoveries
and sightingsobserved
in relationto age
of the bird couldbe relatedto the high rate of juvenilemortalityin the
firstmonthsof the dispersalprocess,
duemainly to movementof the young
away from the natal area (Gonz•tlez1989;seealsoFarner 1945, Greenwoodand Harvey 1982).
Additionally,a small percentageof the decreasein the frequencyof
contactsor recoveriesof wing-markedeagleswith time may have been
causedby wearing of the plasticmaterial and consequent
lossof the
marker. However, we have beenunableto quantify suchwear and thus
to correctfiguresfor sucheffect.The longestlifetime of a wing-marker
in this studywas 4 yr 2 mo, within the rangefor other species(average
2-3 yr, exceptionally
up to 10 yr, Kochertet al. 1983). In generalin-
Vol.60,No.3
Dispersal
ofSpanish
Imperial
Eagles
[377
dividual wing-markers have provedto be of great help in studyingthe
dispersalof youngSpanishImperial Eagles.As has alreadybeenshown
for otherbirds of prey (seereview in Young and Kochert1987).
There is evidencethat pointsto territorial behavioras the main cause
for juveniledispersal(Newton 1979). The fact that youngfrom Dofiana,
the area with the highestbreedingdensity,dispersefurthest,is in accordancewith this hypothesisand agreeswith the resultsdescribedfor other
species
(Myers and Krebs1971, Watsonand Moss 1970). In the Spanish
Imperial Eagle youngthat dispersedsufferedhighermortality ratesthan
thosethat remainedin the natal area (Gonzfilez1989). The probability
of survivalof theseindividualsisshortedby thecostsandrisksof dispersion
"per se" and the lack of familiarity with the new areas they explore
(Greewoodand Harvey 1976, Jenkinset al. 1963, Krebs 1971). However,
the dispersalpatternof Figure 3 suggests
that at leastsomeof the young
were successful
in findingareasof high foodavailability.In theseareas
the youngsettledfor severalmonths,exploitingthe extraordinaryabundanceof rabbits (Oryctolagus
cuniculus)(ICONA 1983), which is the
main prey of this species(Gonzfilez1989).
The decreasein the mean dispersaldistanceafter the bird is 8 mo old
is surely related to philopatry, which is commonamongbirds of prey
(Newton 1975, 1979, Osterlof 1977). Long distancedispersalmust facilitate geneticexchange(Greenwoodand Harvey 1976, 1982). In the
SpanishImperial Eaglejuvenilebirdsmay reachareaswhere otherbreeding populationsexist and sometimesremain there to breed,as evidenced
by an adult bird found breedingin an area different from its place of
birth. This suggeststhat there must exist somegeneticexchangeamong
the variousbreedinggroupsof the SpanishImperial Eagle populationin
Spain, thoughthe magnitudeof this interchangeis still unknown.
ACKNOWLEDGMENTS
We are indebtedespeciallyto F. Hiraldo for his encouragementand supportand to J.
Garzon, M. Caballero, T. Gullick, J. Diaz, A. Muõica, F. Palacios,J. J. Hernandez, L.
Diaz-Ambrona,J. Guzman,J. Jimenez,A. Sanchez,J. M. Salinas,J. Marcos, M. Alvarez,
J. Alonsoand the staff of National Park of Dofiana, EstacionBioloõicade Dofiana, Unidad
de Zooloõia Aplicada, Museo Nacional de Ciencias Naturales, Direccion Gral. Medio
AmbienteExtremadura,for the valuablefield assistance
provided.We thank also C. Aikin
for help in the translationof the text. This study was a cooperativeprojectof ICONA and
CSIC, with financial support from ICONA and DGICYT projectNo. 9261.
LITERATURE
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1989
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