Biological Conservation 1992, 59, 45-50 • : • . .

Nesting habitat selection by the Spanish imperial eagle Aquila adalberti

Luis M. Gonz~lez ICONA, Servicio de Vida Silvestre, Gran Via San Francisco, 435, Madrid 28005, Spain

Javier Bustamante & Fernando Hiraldo Estaci6n Biol6gica de Do~ana, Consejo Superior de Investigaciones Cientificas,

Pabellbn del Per~, Avda. Maria Luisa s/n, Sevilla 41013, Spain

(Received 16 November 1990; revised version received 5 April 1991; accepted 9 May 1991)

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.

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 popula- tions 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 al., 1981; Andrew & Mosher, 1982; Reynolds et al., 1982; Gilmer & Stewart, 1984; Rich, 1986; Kost- rzewa, 1987).

The Spanish imperial eagle Aqui la adalberti Brehm 1861, currently considered as a separate species from the Eastern imperial eagle Aquila heliaca (Hiraldo et al., 1976; Collar & Andrews, 1988; Gonzfilez et al., 1989), is one of the rarest birds of prey at a worldwide level and is considered

Biological Conservation 0006-3207/92/$05.00 © 1992 Elsevier Science Publishers Ltd.

45

'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 (Gonzfilez et al., 1987). Factors influencing the present distribution of the Spanish imperial eagle have recently been studied (Gonz/dez et al., 1990), but nesting habitat has been described only qualitatively (Valverde, 1960; Garz6n, 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.

METHODS

Two national censuses of the Spanish imperial eagle have been conducted, in 1971-74 (Garz6n, 1972, 1974) and in 1981-86 (Gonz/dez et al., 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 pre- pared by the Agriculture Ministry. We then evaluated the habitat within a radius of 3.25 km

46 Luis M. GonzMez, Javier Bustamante, Fernando Hiraldo

from each nest, i.e. half the average distance between nests of neighbouring pairs, following the methods of Grubb (1976), Howell et al. (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 26 712 km 2) in which the species had been located (Gonzfilez 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 6"5 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,

Table 1. Variables used to characterize

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

the nest sites of the Spanish imperial eagles

Mnemonic code Meaning

1. DINBUIL ~ 2. DVILL a 3. DPAVRO a 4. DUNPAVRO a 5. HEIGHT a 6. TOPIND a

7. KPAVRO ~ 8. KEL a 9. INHAB a

10. DTB b 11. TCOV b 12. PATIND b

13. QUEFOR b 14. SABFOR b

15. CONFOR b 16. OTHFOR b

17. SCRUB b 18. OPLAN b

19. INACC

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 0"5 km. Kilometres of electric power lines in the circular sampling area, to the nearest 0.5 km. Number of inhabitants in a radius of 3"25 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 × TOPIND.

All horizontal distances measured in kilometres to the nearest 0"05km unless stated otherwise; percentage values to 1%. From topographic maps.

b From farming and land use maps. c 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.

Spanish imperial eagle 47

BMDP (Dixon & Brown, 1983) to identify the set of variables which best separated nest sites and random sites.

RESULTS

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 build- ings (DINBUIL), and villages (DVILL).

In the stepwise discriminant analysis nest sites and random sites were best distinguished by the follow- ing relationships:

Nest sites = 1.806 78 + 0.042 90 KPAVRO

+ 0-153 68 TOPIND

Random sites = 1.761 09 + 0.122 64 KPAVRO

+ 0.115 03 TOPIND

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) (n = 55) and new nest sites (only since 1981)

(n = 34), all nests with both adults (a)

Variable ° Adult pairs Subadult pairs

.~ _+SD .~ + S D

D I N B U I L * 1"8 0"9 1'3 0"7 DVILL 6"9 4"1 5'5 3"3 D P A V R O 3"9 2"8 3'6 2"8 D U N P A V R O * * * + l '0 0"9 0'5 0"3 H E I G H T 679"8 436"7 554'7 325"2 T O P I N D 15"3 9"1 11"4 9"0 K P A V R O + 4"0 5"5 5"7 10"6 KEL* + + + 0"6 2"2 4"2 5"9 I N H A B + + + 290"0 929"0 1 692'0 5 761"0

4~ DTB 0"4 0"7 0"6 0"7 4~ TCOB + + 4 I '0 35"0 45"0 28"0

P A T I N D * 12"6 7'2 17"3 8"4 # Q U E F O R + + + 7"6 15"1 20"6 24'6

S A B F O R + 23"9 28"0 17"9 17"0 # C O N F O R * * + + + 20"1 25"5 5"9 14"6

O T H F O R 2"4 7"5 2"9 5'9 # S C R U B * * + + 25'7 22"0 15'0 11"9 ~ O P L A N + + 16"7 18"6 32"0 30"3

INACC* 54"0 30"7 37"0 24" I

Table 2. Comparison of 19 habitat variables (means and standard deviations) between 108 nest sites and 108 random sites

Variables a Nest sites Random sites

)? _+SD k _+SD

D I N B U I L * * 1"74 0"91 1"38 1"02 DVILL* 6"65 3"98 5"32 3"86 DPAVRO*** 3'87 2"79 2"66 2"54 D U N P A V R O * 0"87 0"81 0"64 0"75 H E I G H T 657'80 420"70 599"70 380"00 T O P I N D * 14'60 9"20 12"00 9"80 KPAVRO*** + + 4'30 6"60 8"60 9"80 KEL*** + + + 1"20 3"40 3"00 6"00 I N H A B 537'00 2565"00 981"00 3076"00 DTB 0"46 0"69 0"60 0"79 T C D B 42"00 33"00 43"00 30"00

~ P A T I N D 13"50 7"60 14"10 8"40 4~QUEFOR 9"90 17"80 7"80 16"20

S A B F O R 22"80 26"40 27"00 31"40 ~ C O N F O R 17"50 24"40 15"70 22"50

O T H F O R + + 2"50 7"20 5'30 16"00 S C R U B 23"70 20"90 18"30 20" 10

~ O P L A N 19"40 21"80 19"70 19"30 INACC* 50"90 30"20 39"80 31"60

For variables marked ~ , n = 88 for nest sites and n = 90 for random sites. Significance of Levene F-test for difference between variances: + + = p < 0.01, + + + = p < 0.001; and of Student t-test for difference between the means, for equal or different variances according to each case: * = p < 0 - 0 5 , ** =p<O'O1, *** = p < 0-001.

(b)

Variable Traditional nests New nests

+ SD .Y + SD

D I N B I L L 1"9 1"0 1"7 0"9 DVILL** 7"8 3"8 5"5 4"1 DPAVRO*** + + + 4"8 3"0 2"5 1"7 D U N P A V R O 1"0 0"9 0'9 0"8 H E I G H T * + + 610"4 480"8 792"1 330"6 T O P I N D + + 14"3 10"1 16"9 7"1 KPAVRO** + + 2"5 4"2 6"4 6"5 KEL + + 0"3 1"4 1 "0 3"0 INHAB* 4- + + 92"0 480"0 611'0 1 384"0

~: DTB 0"4 0"7 0"4 0"7 ~# TCOB 36"0 35"0 49"0 34"0

P A T I N D * + 11"2 6"0 14'8 8"3 ~ Q U E F O R 7"6 14"9 7'5 15"6

S A B F O R 26"6 27"6 19"9 28"7 ~ C O N F O R * * + 13"5 21"9 30"0 27"5 ~ O T H F O R 3"1 8"5 1"4 8"5

S C R U B 25"7 22"0 25"5 22"4 ~ O P L A N + + 19"0 22"1 13"1 11"2 ~: I N A C C + 50"6 33"4 58"9 26"0

° 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 < 0"5, + + = p < 0.01, + + + = p < 0.001; and Student t-test for difference between the means, for equal or different variances according to each case: * = p < 0-05, ** = p < 0.01, *** = p < 0.001.

48 Luis M. GonzMez, Javier Bustamante, Fernando Hiraldo

We found that 61.4% of the nest sites and 63.3% of the random sites were correctly classified. A jack-knife classification reduced the correct classifica- tion of nest sites to 60.2%, but maintained that of random sites at 63.3%; the kappa statistic (Titus et al., 1984) showed a classification rate 24% better than chance (kappa -- 0-2358, 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 al. (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 dis- turbance, as in other birds of prey (Grubb, 1976; Andrew & Mosher, 1982; Kostrzewa, 1987).

The 61.4% correct classification i n t h e disCrimi- nant 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 observa- tions, which showed that the population is growing in marginal habitats around tra- ditional nesting areas (Gonzfilez et al., 1987). This expansion strategy seems to be moti- vated by the species' philopatric behaviour; of 10 eagle chicks ringed and recovered as breeding adults, nine subsequently nested < 10 km from their birthplace and only one 80km away (ICONA Ringing Center and Dofiana Biological Station ringing files). We do not think these data are biased as ringing has been undertaken across the whole distri- bution 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 et al. (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 49

cause of death in birds of prey (Olendorff, 1981) and was the most common cause of non-natural death in the Spanish imperial eagle (50.1% of known causes, n=42) , followed by shooting (28-7%) in the period 1981-1988 (Gonzfilez, 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 (New- ton, 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 i'nformation in this paper and in a previous one on the factors conditioning the eagle's present distribution (Gonzfilez et al., 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 Olendorff (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 & Garz6n, 1973), should be released in other less disturbed areas formerly occupied by the species.

ACKNOWLEDGEMENTS

Special thanks go to J. Garz6n, Jos~ L. Gonzfilez, B. Heredia, F. Palacios and the staff of Unidad de

Zoologia Aplicada INIA (Madrid), the Estaci6n Biol6gica de Dofiana, CSIC (Sevilla) and Parque Nacional de Dofiana (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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