Nesting habitat selection by the Spanish
imperial eagle Aquila adalberti
Luis M. Gonzalez
ICONA, Servicio de Vida Silvestre, Gran Via San Francisco, 435, Madrid 28005, Spain
Javier Bustamante & Fernando Hiraldo
Estación Biológica de Doñana, Consejo Superior de Investigaciones Cient(ficas,
Pabellón del Peru, Avda. Maria Luisa s/n, Sevilla 41013, Spain
()
Nesting habitat selection by the Spanish imperial eagle Aquila adalberti was
quantitatively assessed. Nest sites chosen did not differ from the available habitat
with regard to physiography and vegetation, but the eagles tended to avoid areas
affected by human disturbance. Nest sites used by subadults and those of adults that
were recently established tended to be in more disturbed areas than those used by
adults or in traditional nesting localities. Management recommendations for more
effective conservation of the eagle’s habitat are discussed.
‘endangered’ in the IUCN Red Data Book (King,
1981; Wilcox, 1988). The current world population is
estimated at just over a hundred pairs, all in the
Iberian Peninsula (Gonzalez et al., 1987). Factors
influencing the present distribution of the Spanish
imperial eagle have recently been studied (Gonzalez
et a!., 1990), but nesting habitat has been described
only qualitatively (Valverde, 1960; Garzón, 1974;
Meyburg, 1975). In this paper we provide a
quantitative description of their nesting habitat and
test for differences between available and selected
habitat, between pairs of adult and subadult
breeders, and between traditional and new nest sites.
INTRODUCTION
One of the most important causes of species decline
is the loss or alteration of habitat (Greenway, 1967;
Temple, 1978; King, 1981). Snyder and Snyder (1975)
drew attention in the United States to a progressive
loss of suitable habitat for many species of birds of
prey, and suggested that habitat preservation was of
prime importance in maintaining raptor populations at an acceptable size. This view coincides with
that of various authors in other areas (Newton, 1979;
Steyn, 1983). Since then, numerous studies on the
conservation and management of birds of prey
threatened by local or global extinction have
quantified data on nest sites (Grubb, 1976; Morris,
1980; Bednarz & Dinsmore, 1981; Newton et a!.,
1981; Andrew & Mosher, 1982; Reynolds et a!.,
1982; Gilmer & Stewart, 1984; Rich, 1986; Kost
rzewa, 1987).
The Spanish imperial eagle Aquila adalberti
Brehm 1861, currently considered as a separate
species from the Eastern imperial eagle Aquila
heliaca (Hiraldo et a!., 1976; Collar & Andrews,
1988; Gonzalez eta!., 1989), is one of the rarest birds
of prey at a worldwide level and is considered
METHODS
Two national censuses of the Spanish imperial eagle
have been conducted, in 1971—74 (Garzón, 1972,
1974) and in 1981—86 (Gonzalez et a!., 1987). The
nests recorded during the second census were used in
the present study.
A total of 108 nest sites, belonging to at least 104
different breeding pairs, were marked on 1:50000
topographic maps prepared by the Spanish Army
Cartographic Service and on land use maps prepared by the Agriculture Ministry. We then
evaluated the habitat within a radius of 325 km
45
from each nest, i.e. half the average distance between
nests of neighbouring pairs, following the methods
of Grubb (1976), Howell et a!. (1978), Bednarz and
Dinsmore (1981), Gilmer and Stewart (1984) and
Rich (1986).
In addition 108 random sites were sampled to
evaluate habitat available to the species, using the 53
sheets of the ‘L’ series 1:50 000 topographic map of
Spain (covering an area of 26712km2) in which the
species had been located (Gonzalez et al., 1987). To
avoid bias due to differences in habitat and level of
human influence among the various breeding areas,
random sampling was stratified, the number of
random sites included on each map sheet being equal
to that of previously marked nest sites. Random sites
were located on the maps using random generation
of coordinates with a calculator, and were at least
65 km from any nest site. Since the eagle nests in
trees, random sites which fell in unwooded areas
(farmland, uncultivated land, meadows, rocky
ground, urban centres, reservoirs, etc.) were rejected
(Bednarz & Dinsmore, 1981; Gilmer & Stewart,
1984; Rich, 1986; Speiser & Bosakowski, 1987).
For every nest site and random site we measured
19 variables on the maps (Table 1). As land use maps
were unavailable for some areas, some variables
could not be measured for 20 nest sites and 18
random sites. Land use and habitat were verified by
field visits, averaging five visits per nest site to each
of the 53 map sheets used.
Breeding subadults (3—4 years old) are readily
distinguished by their light brown plumage spotted
with black compared with the black plumage of
adults (Valverde, 1960). Pairs with at least one
subadult breeder were analysed separately from
pairs in which both were adult, in order to test
possible differences in habitat selection.
In pairs where both members were adults we also
analysed separately those breeding at ‘traditional’,
i.e. those occupied during both censuses, and at ‘new’
nest sites, occupied only since 1981.
Mean values for nest site and random site
variables were compared using t-tests. A stepwise
discriminant function analysis was conducted using
Table 1. Variables used to characterize the nest sites of the Spanish imperial eagles
Mnemonic code
1.
2.
3.
4.
5.
6.
DINBUIL”
DVILL”
DPAVRO
DUNPAVRO’
HEIGHT’
TOPIND
7. KPAVRO’
8. KEL°
9. INHAW
10. DTW’
11. TCOV”
12. PATINDb
13. QUEFORb
14. SABFORb
15. CONFORb
16. OTHFORb
17. SCRUBb
18. OPLANb
19. INACC
Meaning
Distance from nest to nearest inhabited building.
Distance from nest to nearest urban centre.
Distance from nest to nearest paved road.
Distance from nest to nearest unpaved road passable by vehicle.
Height of nest above sea-level (m).
Topographic irregularity index. Total number of 20 m contour lines, cut by two lines diametric to the
sampling circle in the directions N—S and E—W.
Kilometres of paved roads in the circular sampling area, to the nearest 05 km.
Kilometres of electric power lines in the circular sampling area, to the nearest 05 km.
Number of inhabitants in a radius of 325 km around the nest, calculated from the number of inhabited
buildings and the proportion of urban centres included in this radius, measured on the topographic map
(20 inhabitants precision).
Distance from nest to the border of wooded area.
Percentage of tree cover in wooded area housing the nest.
Habitat patchiness index, number of times two lines diametric to the sampling circle in the directions N—S
and E—W cut the limit of zones with different agricultural use or different vegetation.
Percentage of surface covered by dense oak forest Quercus spp. (cover >70%).
Percentage of the surface covered by savannah-like forest (pasture and scattered trees): Quercus spp.,
Olea europaea and occasionally Pinus spp., with canopy cover <70% and ground cover with pasture
and cultivated farmland.
Percentage of the surface covered by coniferous forest.
Percentage of surface covered by other species of trees: predominantly Eucalyptus, Fraxinus and
Populus.
Percentage of surface covered by mediterranean scrubland.
Percentage of surface occupied by open land, herbaceous formations or ground without vegetation
(pasture, non-forested cultivated lands, meadows and rocky ground).C
Inaccessibility index, an estimate of difficulty of access by foot in the area calculated as a function of the
steepness of the relief and the surface of scrubland according to the formula INACC = SCRUB +
2 x TOPIND.
All horizontal distances measured in kilometres to the nearest 005 km unless stated otherwise; percentage values to 1%.
From topographic maps.
1’From farming and land use maps.
To avoid redundancy in the discriminant analysis the habitat unsuitable to the eagle (urban centres, reservoirs) is not considered as an
independent variable, being the amount lacking to 100% of the surface once variables 13 to 18 are added.
C
BMDP (Dixon & Brown, 1983) to identify the set of
variables which best separated nest sites and random
sites.
RESULTS
Table 3. Comparison of 19 nest site variables (means and
standard deviations) between (a) adult pairs (n = 89) and pairs
with at least one subadult (n = 19); (b) traditional nest sites
(known since 1971) (a = 55) and new nest sites (only since 1981)
(a = 34), all nests with both adults
(a)
Variable0
Nesting habitat did not differ from random habitat
in any of the variables related to vegetation structure
(Table 2). The only statistically significant differences
between the two groups could be found in those
variables relating to the physical environment and
degree of human influence. Nests were in more
rugged terrain (TOPIND), and in more inaccessible
areas (INACC), with less paved roads (KPAVRO)
and power lines (KEL), than random sites. Nests
were also located further away from paths
(DUNPAVRO), roads (DPAVRO), inhabited buildings (DINBUIL), and villages (DVILL).
In the stepwise discriminant analysis nest sites and
random sites were best distinguished by the following relationships:
Nest sites = 1806 78 + 0042 90 KPAVRO
+ 0153 68 TOPIND
Adult pairs
DINBUIL*
DVILL
DPAVRO
DuNpAvRO***+
HEIGHT
TOPIND
KPAVRO+
KEL* + + +
INHAB + + +
*DTB
*TCOB+ +
*PATIND*
*QUEFOR+ + +
*SABFOR+
*CONFOR**+ + +
*OTHFOR
*SCRUB**+ +
*OPLAN+ +
*INACC*
Subadult pairs
K
+SD
18
69
39
[0
679•8
0•9
41
28
09
4367
91
153
40
06
29oo
04
4[0
126
76
23•9
201
24
257
167
54Q
—
K
13
55
36
05
5547
114
55
57
22
9290
07
350
72
151
280
42
1 6920
06
450
173
206
179
255
75
220
186
307
59
29
150
320
370
+SD
—
07
3.3
28
03
3252
90
106
59
5 76[0
07
2&0
84
246
170
146
59
1F9
303
24I
Random sites = 1761 09 + 0122 64 KPAVRO
+011503 TOPIND
Table 2. Comparison of 19 habitat variables (means and
standard deviations) between 108 nest sites and 108 random sites
(b)
Variable
VariabIes
Nest sites
X
DINBUIL**
DVILL
DPAVRO***
DUNPAVRO*
HEIGHT
TOPIND*
KPAVRO***+ +
KEL***+ + +
INHAB
DTB
*
*TCDB
*PATIND
*QUEFOR
*SABFOR
*CONFOR
*OTHFOR+ +
SCRUB
*
*OPLAN
INACC*
*
±SD
174
091
&65
398
387
279
087
081
65780 42070
1460
920
430
660
F20
340
53700 2565{)0
046
069
4200
3300
760
1350
990
1780
2280
2640
1750
2440
720
250
2370
2090
1940 2F80
5090
3020
2
**=p<O.O1, *** =p<o.ool.
New nests
K
±SD
X
±SD
±SD
138
102
132
386
246
254
044
075
59970 38000
1)00
980
980
860
300
600
98100 3076*
040
079
4300 3000
1410
140
740
2700
1170
130
1130
1970
3980
1620
3140
22*
1600
2010
1930
3160
variables marked *, n = 88 for nest sites and n =90 for
random sites. Significance of Levene F-test for difference
between variances: + + =p <01)1, + + + =p <01)01; and of
Student 1-test for difference between the means, for equal or
different variances according to each case: * =p <01)5,
°
Traditional nests
Random sites
DINBILL
DVILL
DPAVRW+ + +
DUNPAVRO
HEIGHT++
TOMND+ +
KPAVRO+ +
KBL++
INnAr+++
+DTB
+TCDB
+PATIND+
tQUBPOR
+SABFOR
+CQNFOR+
+U1HPOR
+SCRUB
4’CPLAN++
•INACC+
a
19
78
48
1-0
6104
143
25
0-3
92-0
0-4
36-0
112
7-6
26-6
13-5
3-1
25-7
19-0
50-6
10
[7
38
30
09
4808
101
4-2
1-4
4800
07
35-0
6-0
14-9
27-6
21-9
8-5
22-0
22-1
33-4
55
25
0-9
792-1
16-9
6-4
1-0
611-0
0-4
49-0
14-8
7-5
19-9
30-0
1-4
25-5
13-1
58-9
09
41
17
0-8
330-6
7-1
6-5
3-0
13840
0-7
340
8-3
15-6
28-7
27-5
8-5
22-4
11-2
260
For variables marked *,the samples are n = 72 and n = 16 for
adults—subadults, respectively, and n =43 and n = 29 for
traditional—new, respectively. Significance of Levene F-test for
difference between variances: + =p <05, + + =p <01)1,
+ + + = p < 01)01; and Student 1-test for difference between
the means, for equal or different variances according to each
case:*=p<01)5,**=p<01)1,***=p<0.001.
48
Luis M. Gonzalez, Javier Bustamanie, Fernando Hiraldo
We found that 614% of the nest sites and 633%
of the random sites were correctly classified. A
jack-knife classification reduced the correct classification of nest sites to 6O2%, but maintained that of
random sites at 63-3%; the kappa statistic (Titus et
a!., 1984) showed a classification rate 24% better
than chance (kappa = O2358, Z = 3-15; p <0-01).
Comparison of the nesting habitat of adult pairs
and pairs with at least one subadult member (Table
3) showed that subadult pairs occurred in areas with
greater human influence, their nest sites being closer
to villages and roads, in more accessible areas and
with more kilometres of power lines. Confining the
analysis to adult pairs only, a similar tendency was
obtained when comparing the habitats of ‘tra­
ditional’ and ‘new’ nests (Table 3). New nest sites
were in areas with significantly more human
inhabitants and kilometres of roads, and tended to
be located closer to villages and roads than were
traditional ones.
DISCUSSION
Unlike other species of raptors described by
McGowan (1975), Reynolds et at. (1982), Bednarz
and Dinsmore (1982), Janes (1985) and Rich (1986),
the Spanish imperial eagle does not seem to prefer
one habitat rather than another in terms of cover
and vegetation: nest selection in this respect reflects
the proportions of habitat found in the area, and the
eagle does not appear to seek out areas more
forested than the average. This may be partly due to
the fact that the population we studied is located in
an area influenced by centuries of human activity
(see Bauer, 1981). Given the eagle’s size and mobility
in relation to the size of the patch (PATIND values
much greater than 1, Table 2), a tendency towards a
particular habitat specialization would appear
difficult (see MacArthur & Pianka, 1966), and the
variables most associated with the choice of nesting
habitat indicate attempts to avoid human disturbance, as in other birds of prey (Grubb, 1976;
Andrew & Mosher, 1982; Kostrzewa, 1987).
The 61-4% correct classification inthe discriminant function was only 24% better than expected by
chance. We believe that the lack of a higher
discrimination was due to the heterogeneity of the
sample studied, which included breeding areas with
very different landscapes, and to individual variation
in the birds’ general intolerance to the presence of
humans, as occurs with other large birds of prey
(Grubb, 1976).
The fact that pairs with at least one subadult were
found in more disturbed habitats can be attributed
to several possible causes:
(1) These pairs were less selective in choosing
their nesting habitats.
This cannot be totally excluded as there
were no significant differences for such pairs
between nest sites and nearby random sites
(n
31) for any of the 19 variables examined.
We do not, however, consider that subadults
are less selective; in 90% of the cases one
member of the pair was an adult and pairs
formed only by adults at ‘new’ nest sites also
occurred in more disturbed habitats.
(2) They were mainly new pairs, choosing
marginal habitat because less disturbed
habitat was saturated by traditional adult
pairs.
This was suggested by our field observations, which showed that the population is
growing in marginal habitats around traditional nesting areas (Gonzalez ci al., 1987).
This expansion strategy seems to be motivated by the species’ philopatric behaviour;
of 10 eagle chicks ringed and recovered as
breeding adults, nine subsequently nested
<10km from their birthplace and only one
80km away (ICONA Ringing Center and
Doñana Biological Station ringing files). We
do not think these data are biased as ringing
has been undertaken across the whole distribution area for the last ten years. This
behaviour, also shown in other species of
birds of prey (Newton, 1979), would favour
the saturation of existing nesting nuclei
(Diamond, 1976, 1984) and the occupation of
surrounding areas, even if the habitat is
disturbed, before recolonization of more
suitable habitats at greater distances. In fact,
new pairs of adults are also settling in such
areas that are more disturbed than average
(Table 3).
(3) Turnover rates of pair members in these
more disturbed habitats were higher, and this
increases the frequency of a subadult being
incorporated into the breeding population.
This has been suggested for the golden
eagle Aquila chrysaetos by Steenhof ci at.
(1983), and is supported here by the fact that
there are significantly more kilometres of
power lines (KEL) in the breeding territories
of subadults. Electrocution is a frequent
Spanish imperial eagle
cause of death in birds of prey (Olendorif,
1981) and was the most common cause of
non-natural death in the Spanish imperial
eagle (501% of known causes, n = 42),
followed by shooting (287%) in the period
1981—1988 (Gonzalez, 1989). The predicted
increase in the mortality of breeding birds in
habitats with a higher level of human
disturbance increases the probability that
subadults will breed in these areas (Newton, 1979). We believe that the saturation of
safer nest sites in the traditional breeding
areas, and the increased mortality of breeders
in more disturbed habitats, are responsible
for the subadults’ choice of nesting habitat
rather than simply poor judgement on the
part of these birds.
Management implications
Management of Spanish imperial eagle nesting
habitat should include an inventory of existing and
potential habitats. This can be accomplished in part
by using the information in this paper and in a
previous one on the factors conditioning the eagle’s
present distribution (Gonzalez et a!., 1990).
Construction of new forest paths and power lines
in the actual nesting habitat should be avoided.
Power lines already crossing these should be
modified to reduce electrocution risks, following the
recommendations made by Olendorif (1981).
Since this species appears to be sensitive to
disturbance, we recommend that all public use,
forestry works, etc. should be reduced within a
radius of 500 m around nests from the initiation of
breeding until young fledge—from February to
August. Planting should be carried out along some
of the existing forest paths to reduce or eliminate the
eagle’s visual contact with possible disturbance
(Andrew & Mosher, 1982).
As the population seems to be expanding around
existing nesting nuclei where the remaining habitat is
affected by high levels of human disturbance, we
suggest that chicks produced in captivity, or taken
from nests with large broods (3—4 chicks) to reduce
mortality caused by cainism (Meyburg & Garzón,
1973), should be released in other less disturbed
areas formerly occupied by the species.
ACKNOWLEDGEMENTS
Special thanks go to J. Garzón, José L. Gonzalez, B.
Heredia, F. Palacios and the staff of Unidad de
49
Zoologia Aplicada INIA (Madrid), the Estación
Biológica de Doñana, CSIC (Sevilla) and Parque
Nacional de Doñana (ICONA) for their help during
the field work. We received helpful review comments
from K. Titus and two anonymous referees. The
study was financially supported by ICONA and the
DGICYT Projects No. 2007 and PB87-0405.
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