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Article

Selection of Nesting Habitat and Insular Niche Separation of Two Sympatric Aquila Species

by
Maria Solanou
1,2,
Eirini Trypidaki
1,
Elisavet Georgopoulou
1,
Konstantinos Damianakis
1,
Afroditi Kardamaki
1 and
Stavros M. Xirouchakis
1,*
1
Natural History Museum of Crete, School of Sciences & Engineering, University of Crete, University campus (Knossos), 71409 Heraklion, Greece
2
Institute of Marine Biological Resources and Inland Waters (IMBRIW), Hellenic Centre for Marine Research (HCMR), Former US Base at Gournes, P.O. Box 2214, 71003 Heraklion, Greece
*
Author to whom correspondence should be addressed.
Diversity 2022, 14(12), 1136; https://doi.org/10.3390/d14121136
Revised: 7 December 2022 / Accepted: 15 December 2022 / Published: 18 December 2022
(This article belongs to the Special Issue Conservation and Ecology of Raptors)
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Abstract

:
Aquila chrysaetos and Aquila fasciata are two congeneric eagle species distributed in the Mediterranean region which are supposed to compete for similar breeding and foraging resources. In the present study, bioclimatic, topographic, and human-related habitat parameters were investigated for 64 and 75 nest sites of Golden and Bonelli’s eagles, respectively. The nests were found during fieldwork undertaken from 1995–2020. Overall, the habitat parameters that best discriminated nest site selection were associated with elevation, temperature, and land use with topographic variables being most powerful for niche separation. Univariate analysis, regression, and species distribution modeling identified a strong association of the species with altitude pinpointing the Golden eagle’s mountainous and continental character and the Bonelli’s eagle being a lowland and coastal species. Golden eagle nests were situated away from human settlements on steep cliffs in higher altitude areas with transitional woodland-shrub vegetation. In contrast Bonelli’s eagle nests were located on low-altitude warmer zones, closer to the coast and human settlements with more natural grasslands in their vicinity. The ecological niche separation of the two species was best described by altitude and temperature, though no clear-cut evidence was detected for their competitive exclusion. Inter-specific nearest neighboring distance was found statistically significant only for the Golden eagle which seems to be less tolerant in its co-existence with the Bonelli’s eagle. Conservation measures for both species should target territories under human pressure, though more research should focus on the species range use and habitat heterogeneity within overlapping territories.

1. Introduction

Eagles are long-lived raptors at the top of the food chain occupying sizeable home ranges in order to meet their ecological requirements; a fact the makes their conservation and management quite difficult particularly within human-modified landscapes [1,2]. Aquila species are quite vulnerable to land use changes, rural and urban development, and infrastructure facilities, all of which result in the loss of suitable habitat affecting their population dynamics [3,4,5]. However, apart from human induced factors a major source of habitat exclusion might be the competitive interactions among species with similar ecological niches and overlapping distributions [6,7,8,9]. In the Mediterranean region this is the case of the Golden (Aquila chrysaetos) and the Bonelli’s eagle (Aquila fasciata) who share similar nesting and foraging habitat [10,11] with potential constraints on the breeding density of the latter [12,13,14,15]. Interspecific competition has been taken into account in many conservation efforts for the recovery of the Bonelli’s eagle in Europe as many of the species deserted territories have not been recolonized due to their usurpation by Golden eagles [14,15,16,17].
In Greece, the Golden eagle is mainly distributed on the mainland and the island of Crete with an estimated population of 100–150 breeding pairs [18,19,20]. In contrast the Bonelli’s eagle is restricted to southern continental Greece (e.g., Peloponnese) and the Aegean islands including Crete [18] with a population numbering no more than 140 breeding pairs. Both species are enlisted in the Greek Red Data Book as “Endangered” and “Vulnerable”, respectively, due to negative population trends that have been mainly caused by human persecution, secondary poisoning, collision with energy infrastructure and land use changes [20].The island of Crete harbors viable and relatively stable populations of the species [21,22] and along with Sicily constitutes the only Mediterranean island where they co-exist [23]. Both species are cliff nesting and depend on similar prey, i.e., medium to small size mammals and birds [20,21].
According to the theory of competitive exclusion, sympatric species with similar life-history and ecological traits would exhibit some form of distinction in ecological niche and resource partitioning in space or time or dietary segregation [24,25,26,27,28]. In this context and given that island raptors occupy niches of wider breadth compared to their mainland counterparts [29,30,31], Crete is a good model area to quantify habitat features that facilitate niche separation of the two species. Moreover, understanding the processes undergoing their spatial distribution on the island and their demand on natural resources are pivotal for supporting effective management policies for both species at a national level.
In the aforementioned framework the objectives of the current study were to: (a) census the eagle territories on the island of Crete giving special emphasis on defining the overlapping ones, (b) investigate the environmental and anthropogenic factors that influence nest site selection or limit territory occupancy, (c) asses niche similarity and partitioning within eagle breeding range, and (d) construct habitat suitability maps for depicting potential nesting areas for recolonization. The ultimate aim was to elucidate the ecological mechanism for the coexistence of the Golden and the Bonelli’s eagles on Crete and provide baseline guidance for the efficient conservation planning and simultaneous management for their populations.

2. Materials and Methods

2.1. Study Area

Crete is the largest island of Greece, with an area of 8261 km2 and a length from east to west of 260 km. Its width ranges between 12 and 60 km and its terrain is characterized by mountainous areas (60%) that are crisscrossed by cracks, numerous gorges (>120), rocky outcrops, and steep cliffs [32]. The three major massifs extending from west to east are the Lefka Ori mountain range (2450 m), Mount Psiloritis (2457 m), and Mount Dikti (2100 m). The island’s overall terrain consists of some plains and many hills covered with extensive vineyards and olive groves. Depending on the longitude and the altitude, the annual rainfall ranges between 400 and 2000 mm extending over 70–130 rainy days per year. The average temperature in winter ranges between 11–16 °C while in summer between 21–28 °C [33]. Winters are mild and usually wet, while summers are dry and hot. Snow is common in the mountains, and can persist from late November until early May. Despite having been altered by humans for at least 3000 years, the predominant vegetation is dry-leafed maquis and the cushiony shrubs, i.e., phrygana [34].

2.2. Fieldwork

The study area, namely the entire island of Crete, was divided to 330 sample units of 5 × 5 km, namely quadrats of 25 km2. Nest searches for both eagle species were conducted systematically from 1995 to 1999 in 165 quadrats covering 50% of the island’s surface. Fieldwork constituted of midday surveys carried out during mid-November–late March and observations were made from a vehicle moving at a mean speed of 35 km per hour [35]. Fieldwork usually started two hours after sunrise and ended at 16:00 pm, although mean effective time was 6.5 h per day. All eagles observed with unaided eye on both sides of the road were recorded and identified by two observers [36,37,38]. For each sighting, eagle position was recorded on a 1:50,000 scale map (Hellenic Military Geographical Service) and notes were kept on date, time, locality, altitude, number of individuals, and activity codes [1,35,39]. Additional random surveys were undertaken over the entire island during 2000–2005 and 2015–2020. Territory occupancy was validated by observations made from vantage points in areas with suitable breeding habitat and high eagle use. Cliffs were inspected during the pre-breeding period looking for eagles performing undulating flights, carrying nest material or defending a nesting territory. All eagle nests were found by the use of 10 × 50 binoculars and a 30–60× spotting scope and their geographical position was fixed on a handheld hiking GPS device (Garmin Montana 680 t) on a WGS 84 coordinate reference system. Their position was transferred, digitized, and transformed into UTM coordinates of the Hellenic Geodetic Reference System (Hellenic Mapping & Cadastral Organization, 1995) by the aid of a Geographic Information System (i.e., ArcGIS Pro, ESRI 2021).

2.3. Data Collection & Processing

Environmental and human pressure variables were selected according to their relevance with the species nesting ecology (Table 1). A subset of current climatic variables was designated with regard to the species breeding seasons (January–March) and were obtained from the open access WorldClim database at a resolution of 30 s (https://www.worldclim.org/, accessed on 20 July 2021). Human-related variables were calculated using satellite imagery (http://geodata.gov.gr/; https://www.openstreetmap.org, accessed on 20 July 2021), while topographic variables were produced by the aid of a Digital Elevation Model (DEM-Digital Elevation Model) of Crete with a 50 × 50 m2 resolution (http://srtm.csi.cgiar.org/, accessed on 20 July 2021). The circular variable of “aspect” (0–360°) was transformed into two linear ones, namely “eastness” and “northness”, using cosine and sine, respectively, and their measurements ranged between −1 and 1, with positive values indicating an eastward and northward tilt, correspondingly. The normalized enhanced vegetation index (EVI) [39] was used to represent plant biomass and data for two periods, i.e., Winter (December–February) and Summer (May–July), were extracted from MODIS satellite images at 16 day interval and 250 m resolution. Average EVI values for the years 2000 to 2020 were calculated by the R library MODIStsp [40] with the R 4.1.1 software [41]. Concerning land use variables, the Corine Land Cover database was used covering the years 1980–2000 and 2000–2020 [42,43] in accordance to the main fieldwork data collection periods (https://land.copernicus.eu/pan-european/corine-land-cover, accessed on 20 July 2021). Land cover types were collapsed to the most meaningful ones in biological terms for eagle territories (Table 1) and their percentages were calculated in buffers of 6 km and 4 km around Golden and Bonelli’s eagle nests, respectively. These figures were regarded as the shortest foraging radius of a breeding eagle pair within a hypothetical circular territory and an average home range size of ca. 100 km2 for the Golden eagle and 50 km2 for the Bonelli’s eagle [10,21,44,45,46,47,48,49]. The latter approach was considered to be representative for central place foragers when breeding such as the study species [50] and consistent to the radius of major activity around nest sites reported in the literature [15,51,52]. As a proxy for eagle competition, the nearest neighboring inter-specific Euclidean (horizontal) distance (NNID) was calculated, and was further used in the statistical analysis.

2.4. Statistical Analysis

2.4.1. Nesting Habitat & Territory Overlap

All explanatory variables were extracted for individual eagle nest sites. Univariate comparisons were made by the use of Student’s test at a 0.05 level of statistical significance or two-tailed Mann–Whitney–Wilcoxon test in case the assumption of normality of data distribution was violated [53]. Territory overlap was delineated by using the Predicting Aquila Territory (PAT) model which provides an estimate of home range use based on nest site locations, elevation and terrain [51,54,55,56]. Major assumptions of the model were the excessive use of ridges and rocky outcrops around nest sites and the complete avoidance of human activity areas (i.e., roads and settlements) and marine or freshwater bodies. Relevant spatial data (i.e., polygons) were downloaded from open access databases (http://geodata.gov.gr/, https://www.openstreetmap.org, accessed on 25 July 2021), or were constructed by using a digital elevation model (DEM) of Crete (http://srtm.csi.cgiar.org/, accessed on 25 July 2021). For eagle territories with multiple alternative nests [57], their geometric mean was taken into account. Thiessen polygons were used to establish territorial boundaries between nests, while a maximum radius of 6 km and 4 km were defined around Golden and Bonelli’s nests, respectively, in case of absence of neighboring territories [55,58,59,60]. The grid cells constructed in the ArcGIS Pro environment were given certain values and predictive maps with areas of high eagle use within each territory were produced.

2.4.2. Niche Similarity & Partitioning

An equivalency test was implemented in order to address whether the congeneric eagle species are effectively identical in their predicted realized environmental distributions [61]. At first, two similarity metrics, namely the Schoener’s D [62] and the Warren’s I, were calculated by the ENMTools R library [63] both ranging from 0 (no overlap) to 1 (niche models identical). Subsequently, all occurrence data from both species were pooled into one dataset and were tested by generating new overlapping values via a repeatedly randomization process of one hundred (100) permutations [64]. Empirical overlapping values were finally compared to the critical ones in order to assess the species’ overlap. Namely, empirical values lower than the corresponding empirical ones are indicative of a relatively low overlap and important niche differentiation.

2.4.3. Nest Site Selection & Habitat Suitability

Nest site locations for each species were pooled for the entire study period and filtered by the aid of the spThin R library [65] so as to retain sites at least 1 km apart and avoid spatial autocorrelation issues. For the explanatory variables, two subsets of data were formulated: one consisting by the landcover types and another one with the rest of the environmental and human pressure variables. All variable layers were processed as regular grids at a spatial resolution of 400 × 400 m2 in ArcGIS Pro (ESRI 2021). A logistic regression model was built for each set of explanatory variables, assuming a binomial error distribution and a logit link function for binary response variables (i.e., 1 = nest presence, 0 = nest absence). Absence points were randomly generated at a minimum distance of 1 km apart, excluding points falling at a distance of 3 and 2 km from Golden and Bonelli’s nests, which correspond to their mean core area radius, respectively [15,51,52]. Multicollinearity between explanatory variables (Table 1) was checked by the Variance Inflation Factor (VIF), where only those with values smaller than 10 were retained in the dataset so to avoid overparameterization [53,66,67,68]. Missing values were removed, and the top model’s performance was assessed using k-fold cross validation with five folds [69].
Furthermore, a Maximum Entropy method was applied using the MaxEnt ver. 3.4.4 software [70] by using presence-only data and default settings in order to achieve repeatability. NNID were excluded from the analysis as it was impossible to incorporate pseudoabsence data for them. Overall, 5000 background points were generated for each species and a 5-fold cross-validation process was applied by randomly selecting 75% of them as training and the other 25% as testing data [69]. The technique’s prediction accuracy was evaluated by area under the receiver operating characteristic curve (AUC) metric [71,72]. Values for the parameter indicate high accuracy with a perfect discriminatory ability (i.e., >0.9), moderate accuracy (0.70–0.90), or low accuracy of the model with discrimination no better than random (ca. 0.5) [73]. Finally, the predictors’ importance was evaluated with jackknife analysis and a logistic output was selected because of its extensive use in ecological research and ease of interpretation. In addition, the overlap of the of the habitat suitability maps was checked by converting the probability surface to presence–absence by using the Maximum of sensitivity plus specificity optimalization criterion [70]. This threshold minimizes the mean of error rate for positive and negative observations and determines the optimum cut-off point in the ROC curve and produces binary maps.

3. Results

3.1. Nesting Habitat & Territory Overlap

Overall, 64 and 75 nest sites belonging to Golden and Bonelli’s eagles, respectively, were found on precipitous cliffs. Statistically significant differences were detected between climatic, topographic, and human-related parameters, which described the species nesting sites (Table 2). More specifically, the nesting cliffs of Golden eagles were located in open areas on higher altitude with intermediate woodland-shrub vegetation in their surroundings and received more rainfalls during the breeding season. In addition, the species was found breeding away from human settlements and warm coastal areas, though being more tolerant to temperature variability. In contrast, the Bonelli’s eagle nest sites were located on cliffs of lower altitude in warmer zones, closer to human settlements and the coast, which is covered by various vegetation types and receive irregular rainfalls. However, some of the aforementioned differences became redundant when 13 Golden and 19 Bonelli’s eagle nests (alternative nests included) in nine overlapping territories were examined (Table 3).
In fact, three variables were retained, namely the altitude of the nests and their distance from the nearest road and human settlement, showing that the Bonelli’s eagle might breed closer to urban areas. In addition, two habitat variables distinguished furthermore the two eagle species with the Golden eagle nesting in more open areas with sparse vegetation and the Bonelli’s eagle in areas covered by a higher proportion of cultivations namely olive groves. This pattern was also depicted geographically in the species distribution where they seemed to co-exist in a transitional zone from mountainous semi-natural habitats to human-altered middle lands with extensive farming at their periphery.
Both similarity indices between the nesting habitats of the two species were lower than expected by random as shown by their calculated empirical and permuted critical values. More specifically the D and I metrics were significantly different than their empirical values (A. chrysaetos: 0.80 vs. 0.53 and A. fasciata: 0.90 vs. 0.53) implying that the nesting habitat of the two eagle species are statistically different, even if both of them select similar breeding habitats or nest close to each other. Likewise, the PAT model (Figure 1) showed that grids of high eagle use in overlapping territories accounted for 25% and 44% of those belonging to Golden and Bonelli’s eagles, respectively.

3.2. Nest Site Selection & Habitat Suitability

The binary logistic regression model showed that both species build their nests on steep cliffs away from olive groves although the latter effect was marginally non-significant for the Bonelli’s eagle (Table 4). Furthermore, the Golden eagle exhibited a wider temperature and altitudinal range away from cultivations while in contrast the Bonelli’s eagle that was found breeding on cliff walls in low altitude areas with more vegetation or natural grasslands in their vicinity. The inter-specific nearest neighboring distance was found statistically significant only for the Golden eagle pinpointing an additional important predictor for its nest site selection.
The MaxEnt model predicted that more suitable nesting habitat for the Golden eagle exists on eastern Crete as well as in the periphery of the major massifs of the island (Figure 2a) whereas that of the Bonelli’s is mainly restricted to less mountainous areas and largely on coastal zones (Figure 2b). The prediction accuracy of the model for both species was adequately high (i.e., A. chrysaetos: AUC = 0.884 ± 0.02; A. fasciata: AUC = 0.880 ± 0.03). Furthermore, the inspection of the overlap between binary presence/absence predictive maps showed that 35% and 60% of suitable nesting habitat is uniquely and potentially available for Golden and Bonelli’s eagle nesting, respectively.

4. Discussion

In general, the habitat parameters that best discriminate nest site selection between the two species were associated with elevation, temperature, and land cover type, which most probably reflect prey variety and availability [74]. Both species select vertical precipices for breeding which seems self-explanatory for cliff-nesting raptors, though steep cliffs have been regarded as a prerequisite for territory occupancy in the case of the Bonelli’s eagle [75]. Univariate analysis and regression models revealed that certain habitat features were also favored by the species in the vicinity of their nest sites such as an intermediate woodland to shrubland for the Golden eagle and natural grasslands for the Bonelli’s eagle. These findings should be attributed to the altitudinal differences between the nesting territories of the two species as well as to their main prey items. The Golden eagle nests in mid- and high-altitude areas where bushy and herbaceous vegetation with occasional scattered trees and bare land prevail and its diet consists mainly of brown hares (Lepus europeus), chukar partridges (Alectoris chukar), and wood doves (Columba palumbus), as well as livestock and carrion [18,20,21]. On the contrary, the Bonelli’s eagle nests at lower altitude and closer to the coast, while being smaller in size and nimbler, may also hunt Rock doves (Columba livia), common pigeons, gulls (Larus spp.), corvids, and even passerines (e.g., Turdus spp.) [20]. On the other hand, both species avoid olive groves most likely due to the scarcity of available prey in the understory and the lack of suitable cliffs for nesting or uplift soaring [55,56,76,77]. In general, farmland is suboptimum habitat for both species [2,49,57,78,79] although rural areas extensively exploited has been reported to be beneficial for both [59,80,81]. This was also shown in the present study as the Golden eagle selected transitional zones of woodland to shrubland; a habitat with open spaces that can provide ample feeding opportunities to breeding individuals (e.g., livestock carrion, medium-sized mammals). In the same context, natural grasslands that were selected by the Bonelli’s eagle are habitats of low productivity under moderate human use and encompass rocky areas on steep slopes and patches of natural vegetation that favors breeding and foraging. Similarly, the olive groves selected by the Bonelli’s eagles in overlapping territories are trees cultivated in traditional terraces with patches of natural scrubland in their margins and differ greatly from the monoculture of the plains.
Relevant discrepancies between the two species were also detected for the bioclimatic variables (e.g., temperature, precipitation) prevailing in their nesting habitat. For instance ranges of extreme temperature conditions were more influential on the breeding distribution of the Bonelli’s eagle than its congeneric Golden eagle and most probably were associated with altitudinal preferences [26,82]. In particular the ecological niche separation by temperature might be adjusted by the altitude and has been suggested that reflects the species biogeographical background namely Holarctic for the Golden eagle and Indoafrican for the Bonelli’s eagle [83,84,85].This fact was also advocated in the species overlapping territories at mid-altitude areas where no significant differences were noticed in any of the bioclimatic variables examined.
Regarding human pressure, the nests of Bonelli’s eagles were located closer to human settlements presumably due to their breeding in lower altitude or their tolerance to a certain degree of human presence and the relatively high availability of their favorable prey (e.g., rabbits, pigeons) in altered environments [14,24,57,85,86]. In contrast, the location of Golden eagle nests is in more remote inland areas, although in line with previous research [2,85,87,88,89,90], this should be regarded as a byproduct of direct persecution. The Golden eagle is a top predator on Crete and has been heavily persecuted by stock-breeders because it preys upon newborn lambs. High human-induced mortality is depicted locally in the number of territories near inhabited areas which remain abandoned or occupied by single adult birds or mixed pairs of adult and pre-adult individuals [21,91]. In contrast, this is not the case for the Bonelli’s eagle. This species occupies territories in the vicinity of inhabited areas which are located in narrow inaccessible gorges and coastal cliff faces where eagle-man encounters are rare or impeded by the species cryptic character.
In conclusion topographic variables were demonstrated to be most powerful for niche separation between the study species and most significant in predicting suitable nesting habitat on the island. Respective modeling identified a negative correlation of Bonelli’s eagle with altitude revealing its lowland and coastal character in contrast to the Golden eagle who proved to be a mountainous and continental species. At the same time, the inclusion of other climatic disturbance or land use variables did not improve the predictive power of the model. However, a number of additional influential factors, i.e., prey diversity, density of power lines, or human infrastructures that were not incorporated in the present study could explain distributional gaps and enhance the interpretability of any possible synergies of human activities and inter-specific competition [57,91,92,93,94,95].
The ecological niche separation facilitated by altitude and consequently temperature has been suggested by other studies to be related to inter-specific competition [85,96] or even the local decline of the Bonelli’s eagle [84,97]. In the present study, the two eagle species occupied significantly differential ecological niches selecting distinct nesting habitats at an altitudinal gradient. However, this fact does not inevitably imply an attempt to reduce inter-specific interactions. The PAT and niche similarity analysis did not exclude spatial overlap rather than highlighted differential ecological requirements (e.g., territory size, prey variety, food availability) and close coexistence in 44% of the areas highly used by the Bonelli’s eagle in overlapping territories. Besides the inter-specific nearest neighboring distance was found statistically significant only for the Golden eagle, suggesting that this is the less tolerant species in its co-existence with the Bonelli’s eagle rather than the other way around. Territorial defense and aggression against intruders have been regularly observed during fieldwork and this was always directed from Bonelli’s eagles against Golden eagles. Besides during the study period, no Golden eagle colonized an abandoned Bonelli’s eagle territory although floating non-territorial birds of the former regularly visited vacant nesting cliffs of the latter. This fact could be explained by the low persistence of Golden eagles close to human activities most likely due to direct persecution.
In a conservation perspective, it has been supported that management actions, beneficial for both species, should focus on mountain habitats with intermediate temperature, low human density and high prey diversity [84]. In the current case, this is especially so in rural areas where the landscape is dominated by mixed and natural Mediterranean vegetation such as shrubland and semi-natural grasslands or extensive olive-tree farming. In addition, more research is needed on the species home range use and habitat heterogeneity which could favor their coexistence in overlapping or neighboring territories [29,98,99]. Until then, conservation planning should be a priority in an effort to eliminate mortality factors related to human infrastructure (e.g., colliding to power lines or wind energy facilities) or disturbance from construction and operation activities which may lead to breeding failures or the permanent loss of nesting territories.

Author Contributions

Conceptualization, S.M.X.; methodology, S.M.X., M.S., E.T. and E.G.; software, S.M.X., E.T. and M.S.; validation, E.G.; formal analysis, S.M.X. and M.S.; investigation, S.M.X., A.K. and K.D.; data curation, S.M.X., A.K. and K.D.; writing—original draft preparation, M.S. and E.T.; writing—review and editing, S.M.X., M.S., E.T. and E.G.; supervision, S.M.X.; project administration, S.M.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by LIFE17 NAT/GR/000514, the Green Fund of the Hellenic Ministry of Environment and Energy and the A.G. Leventis Foundation (Project KA 10147 of the Special Account of Research Funds of University of Crete).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Diversity 14 01136 g001 550
Figure 1. Distribution of active and overlapping Golden and Bonelli’s eagle territories in Crete.
Figure 1. Distribution of active and overlapping Golden and Bonelli’s eagle territories in Crete.
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Figure 2. Nesting habitat suitability maps (predicted by a Maximum Entropy approach) and probability of nest site occurrence for Golden and the Bonelli’s eagle on Crete.
Figure 2. Nesting habitat suitability maps (predicted by a Maximum Entropy approach) and probability of nest site occurrence for Golden and the Bonelli’s eagle on Crete.
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Table
Table 1. Environmental data collected for Aquila chrysaetos and Aquila fasciata nesting territories on Crete and used as explanatory variables for species distribution modeling and ecological niche separation.
Table 1. Environmental data collected for Aquila chrysaetos and Aquila fasciata nesting territories on Crete and used as explanatory variables for species distribution modeling and ecological niche separation.
VariablesDescription
ClimateTemperature Seasonality (BIO4): Temperature monthly variation (CV) *
Temperature Annual Range (BIO7): Temperature annual variation (°C)
Winter temperature (BIO11): Mean Temperature of Coldest Quarter (°C) *
Precipitation Seasonality (BIO15): Precipitation monthly variation (CV)
Winter rainfall (BIO19): Precipitation of Coldest Quarter (mm) *
TopographyElevation (m)
Slope (°)
DistSea: Distance to coastline (m)
Elevation (m)
Northness (degrees)
Eastness (degrees)
Vegetation coverEVI winter: EVI December-February
EVI summer: EVI May-July
Land CoverBare land
Forests
Agriculture
Natural grassland
Transitional woodland-shrubland
Sclerophyllous
Olive groves
Water bodies
Pastures
Human infrastructures
CompetitionNNID (m)
Human pressureDistTown: Distance to nearest town (≥10,000 inhabitants) (m)
DistSettlements: Distance to nearest settlement (≤10,000 inhabitants) (m)
DistRoad: Distance to nearest road (m)
* Variables excluded from the GLM analysis due to multicollinearity.
Table
Table 2. Mean values ± SD and range (min-max) of environmental and habitat parameters of univariate analysis of Golden and the Bonelli’s eagle nest sites in Crete. (Significant p values in bold and italic).
Table 2. Mean values ± SD and range (min-max) of environmental and habitat parameters of univariate analysis of Golden and the Bonelli’s eagle nest sites in Crete. (Significant p values in bold and italic).
Aquila chrysaetos (n = 64)Aquila fasciata (n = 75)
VariablesMean ± SDRangeMean ± SDRangep
Temperature Seasonality574.4 ± 15.8541.5–605.43566.4 ± 14.23528.5–597.50.01
Temperature Annual Range22.1 ± 0.720.2–23.422 ± 0.7719.3–23.10.62
Winter temperature8.783 ± 24–11.8210.3 ± 1.316.75–12.250.0001
Precipitation Seasonality84.8 ± 2.181.1–89.585.7 ± 1.4982.8–90.40.007
Winter rainfall386.2 ± 73278–544343.2 ± 61.6251–5260.002
Elevation699.7 ± 329.3152.5–1378396 ± 24356–9833.388 × 10−6
Slope25.4 ± 9.98.15–45.225.47 ± 10.94.7–47.80.97
EVI summer0.21 ± 0.040.13–0.310.21 ± 0.050.13–0.340.64
EVI winter0.19 ± 0.050.1–0.270.22 ± 0.050.11–0.330.0025
DistTown26152 ± 97925536–4590724544 ± 117964327–518420.46
DistSettlements2180 ± 1633707–110371640 ± 1602141–101080.0009
DistRoad444 ± 4310–1887292 ± 2540–100590.14
DistSea6807 ± 4399721–165054271 ± 4149300–197310.0009
Sclerophyllous0.29 ± 0.220–0.880.24 ± 0.240–0.80.1
Olive groves0.06 ± 0.070–0.340.09 ± 0.130–0.630.73
Bare land0.08 ± 0.110–0.490.11 ± 0.20–0.780.18
Forests0.09 ± 0.160–0.720.07 ± 0.140–0.760.19
Agriculture0.06 ± 0.060–0.210.08 ± 0.090–0.330.69
Natural grassland0.32 ± 0.210–0.760.36 ± 0.220–0.790.36
Woodland-shrub0.08 ± 0.10–0330.05 ± 0.090–0.270.007
Table
Table 3. Mean values ± SD and range (min-max) of significantly different environmental and habitat parameters of Golden and the Bonelli’s eagle nest sites on overlapping territories in Crete.
Table 3. Mean values ± SD and range (min-max) of significantly different environmental and habitat parameters of Golden and the Bonelli’s eagle nest sites on overlapping territories in Crete.
Aquila chrysaetosAquila fasciata
VariablesMean ± SDRangeMean ± SDRangep
Elevation812 ± 254435–1264546 ± 242200–9830.005
DistS2285 ± 1400707–58551031 ± 574300–27300.002
DistR530 ± 6040–1887317 ± 2700–8600.04
Olive groves0.05 ± 0.080–0.460.15 ± 0.120–0.360.005
Bare land0.10 ± 0.140–0.310.08 ± 0.200–0.740.02
Table
Table 4. Results of the binary logistic regression models investigating the factors that affect nest site selection by the Golden and the Bonelli’s eagles on Crete.
Table 4. Results of the binary logistic regression models investigating the factors that affect nest site selection by the Golden and the Bonelli’s eagles on Crete.
VariableAquila chrysaetosAquila fasciata
Estimate zpEstimatezp
Temperature Annual Range0.12630.240.811.24672.380.017
Elevation−23 × 10−5−0.10.92−0.007−3.490.0005
Slope0.1433.350.00080.17564.497.1 × 10−6
EVI winter−12.1123−1.630.1−12.7893−2.310.021
NNID29 × 10−52.750.006082 × 10−610.32
Olive groves−9.2023−2.920.0035−3.3295−1.840.065
Agriculture−11.0747−2.740.0062−2.8417−1.280.2
Natural grassland2.52091.420.163.51062.410.02
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Solanou, M.; Trypidaki, E.; Georgopoulou, E.; Damianakis, K.; Kardamaki, A.; Xirouchakis, S.M. Selection of Nesting Habitat and Insular Niche Separation of Two Sympatric Aquila Species. Diversity 2022, 14, 1136. https://doi.org/10.3390/d14121136

AMA Style

Solanou M, Trypidaki E, Georgopoulou E, Damianakis K, Kardamaki A, Xirouchakis SM. Selection of Nesting Habitat and Insular Niche Separation of Two Sympatric Aquila Species. Diversity. 2022; 14(12):1136. https://doi.org/10.3390/d14121136

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Solanou, Maria, Eirini Trypidaki, Elisavet Georgopoulou, Konstantinos Damianakis, Afroditi Kardamaki, and Stavros M. Xirouchakis. 2022. "Selection of Nesting Habitat and Insular Niche Separation of Two Sympatric Aquila Species" Diversity 14, no. 12: 1136. https://doi.org/10.3390/d14121136

APA Style

Solanou, M., Trypidaki, E., Georgopoulou, E., Damianakis, K., Kardamaki, A., & Xirouchakis, S. M. (2022). Selection of Nesting Habitat and Insular Niche Separation of Two Sympatric Aquila Species. Diversity, 14(12), 1136. https://doi.org/10.3390/d14121136

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