Temporally Determinate, but Spatially Consistent Breeding Performance of Lesser Spotted Eagle (Clanga pomarina) Along the Southern Periphery of Its Distribution

Abstract

Breeding performance encompasses offspring production, their survival rate, fertility, overall reproductive outcome, timing of reproduction, and breeding frequency. It varies in raptor species, being affected by different biotic, abiotic, and anthropogenic factors. The Lesser Spotted Eagle is a monogamous, long-lived, slowly reproducing raptor, characterized by site-fidelity and strongly territorial behavior. In this study, we examined data collected over a 10-year period, highlighting the eagles’ main breeding parameters, analyzing whether any of them demonstrated significant trends or spatial or temporal differences over the study period. We also searched for a strict correlation between species breeding density and breeding performance. We found out that the mean occupancy rate of the territories marginally decreased β² = −0.64 ± 0.27, p = 0.047, as this process was clearly visible in Sakar Mnt. (β² = −0.66 ± 0.27, p = 0.038), where in 2022, only 67% of the monitored territories were occupied. The overall mean productivity, breeding success, and breeding frequency did not indicate any trend over the years (p > 0.05). Of all tested breeding parameters, occupancy rate (β² = 0.29 ± 0.14, p = 0.04) and breeding success (β² = −0.12 ± 0.06, p = 0.04) varied yearly, while productivity (β² = 0.25 ± 0.12, p = 0.03) and breeding frequency (β² = 0.27 ± 0.12, p = 0.03) were influenced by density. None of the tested indicators demonstrated significant regional differences, which indicated a temporally determinate, but spatially consistent pattern of breeding performance of the species in this part of its distribution. Recently, the landscape pattern in the south-eastern part of the country was marked by spectacular habitat loss, driven by human activities and natural phenomena, with entire biodiversity facing an uncertain future. Prompt action and urgent decisions are needed to prevent the negative consequences of these imminent threats to the species. Conservation efforts should be focused on the restoration of breeding and foraging habitats. Further research on the response of eagles to the effect of natural (fires) and anthropogenic (habitat transformation) factors, as well as the relationship between breeding performance and different drivers of reproduction, such as diet, weather, habitat features, and presence of intra- and interspecific competitors, would be of crucial significance.

1. Introduction

Breeding performance of animals refers to their success in the reproductive cycle in a given year or during their lifetime. It encompasses various aspects of reproduction, including offspring production, their survival rate, fertility, overall reproductive outcome, timing of reproduction, and breeding frequency. In wildlife, breeding performance can be measured by different parameters such as clutch and brood size, hatching success, the number of occupied territories or breeding pairs, as well as the number of successfully fledged chicks. These parameters vary in raptor species, being influenced by different factors such as habitat, prey availability and abundance, human disturbance, age of breeders, weather, and competition [1,2,3,4,5].

However, breeding performance is a key aspect of the species’ breeding demography, being a tool to follow population dynamics, trends, and structure. It plays a vital role in population growth and decline, as it determines how quickly a species can reproduce and replace its members. Due to their role as top predators in food chains, raptors’ condition is often used as a bioindicator of the status and health of the entire ecosystems [6,7]. Breeding performance of raptors can affect the overall health of an ecosystem, as it influences the structure and function of animal populations contained therein. Therefore, understanding the breeding performance of raptors is crucial for the proper guidance of conservation efforts, particularly with regard to threatened species, as it helps identify factors that may be limiting reproduction and develop strategies to improve it.

Lesser Spotted Eagle (Clanga pomarina) hereafter (LSE) is a monogamous, long-lived, slowly reproducing, k-selected species. This migratory, forest-dwelling, medium-sized raptor is characterized by site-fidelity, territory defending, and occupying year after year. Generally, it produces only one fledgling per successful brood, as reproduction rate fluctuates significantly in years [8,9] and geographical areas [10], and is influenced by food abundance [4,11,12], weather [4,13], land use [9], and human disturbance [14].

Although the population size is considered to be stable, in recent decades the number of mature individuals has been declining in many parts of the species’ range [15]. This medium-sized eagle breeds mainly in Central and Eastern Europe, as Bulgaria is a part of the southern limit of its range [16]. However, the species is mostly present in the eastern part of the country, inhabiting deciduous and mixed forests, where territory selection is determined by the great habitat diversity and the preferences for natural grasslands, agricultural mosaic, as well as the avoidance of large forests and arable land [16,17]. Although the population of the species in Bulgaria is accepted to be stable, a novel threat, such as large landscape transformation of mostly foraging habitats, was recently identified [17]. Therefore, long-term follow-up of the breeding parameters in different regions of the LSE population in the country is crucial to establish its trajectory and make informed decisions for effective species conservation.

In this study we: (1) presented longitudinal data of the LSE main breeding parameters in the country and compared them with other parts of the species’ range; (2) analyzed if any of the examined breeding parameters showed a particular trend over the long-term period and whether the same parameters demonstrated significant spatial or temporal differences; (3) tested if there was any strict correlation between species breeding density and breeding performance.

Finally, we verified whether the LSE population situated at the southern limit of the distribution was regulated by a density-dependent mechanism or through individual adjustments. We hypothesized that if the population was regulated by a density-dependent mechanism, then breeding density would strongly affect breeding performance in a way that eagles breeding in a high-density environment would have a lower value of the tested breeding parameters than those reproducing in low-density conditions. Then, if these regulatory mechanisms were not working, the population should be constrained by territoriality.

2. Materials and Methods

2.1. Study Area

Bulgaria covers an area of 110.994 km² and is located in the eastern part of the Balkan Peninsula. The country falls within the temperate latitudes of 43° N., with a predominantly temperate climate. However, during the period 1991–2020, the transition from a colder to a warmer and/or drier climate affected about 36% of the country’s territory, as the most affected areas were the extreme south-eastern and north-eastern regions [18]. The total area of forests in Bulgaria is approximately 4.2 million hectares, representing some 33% of the country’s territory. Of these, about 3.4 million hectares are covered by forests, making the country one of the top countries in Europe with regard to this indicator.

2.2. Design of the Study

We collected data on the distribution and breeding parameters of the LSE over a period of consecutive years (2015–2024), covering the main areas where the species was found (Figure 1). Annually, we surveyed an average of 48 territories, starting with twenty-seven in 2015 and reaching eighty-one in 2021 (

Table 1. Data of the surveyed the Lesser Spotted Eagle territories in the studied period (2015–2024).

Year	No. of Visited Territories	No. of Monitored Occupied Territories	No. of Monitored Breeding Pairs
2015	27	18	13
2016	47	28	18
2017	46	30	24
2018	44	30	21
2019	43	35	23
2020	47	43	20
2021	81	59	37
2022	50	39	27
2023	48	38	27
2024	46	36	26
TOTAL	479	356	236

). A total of 479 territories were visited during the study period.

The first visit to the studied territories was carried out in the second half of April to establish territory occupation. A territory was considered occupied by a pair if we observed territory defence, nest building and decoration, copulation, or courtship by eagles [16]. In May, all monitored occupied territories were inspected to record active nests and breeding pairs. We considered pairs that laid at least one egg and started incubating as breeding pairs [16,19,20]. Nests were considered active if they were “decorated” with green sprays of foliage, contained incubating birds, nestlings, eggs, or remains of eggshells [9]. However, sometimes eagles bring greenery to more than one nest. In these cases, alternative nests were always searched for near the “decorated” ones. All breeding pairs were visited again in June to establish the presence of chicks. The last visit was carried out from the second half of July to mid-August in order to identify the number of fledglings [16]. If a pair was not seen during at least four visits in a breeding season (April–July) and there was no sign of breeding activity, the territory was considered unoccupied [21]. Annually, in some of the visited occupied territories, there were those where eagles had changed their nest, but the new one was not found during the same breeding season. The outcome of such breeding attempts was then unknown, and these cases were excluded from the analysis.

2.3. Data Analysis

We estimated the following breeding parameters in order to assess the reproductive outcome: (a) productivity (P) as the number of fledglings divided by the number of all occupied nests that we monitored; (b) breeding success (BS) as the number of fledglings divided by the number of all incubating pairs; (c) breeding frequency (BF) as the proportion of nests, where at least one egg was laid, relative to all occupied nests that we monitored [4,16]; and (d) occupancy rate (OR) of territories, calculated as the number of years a territory was occupied, divided by the number of years monitored [22].

To assess whether the tested breeding parameters showed significant trends over the study period or demonstrated significant spatial or temporal differences, we used simple general regression models (GRMs) with sigma-restricted parameterization. Due to the fact that the LSE obligately raise one chick (we had only one case of two successfully raised chicks) we initially tested breeding performance using a binomial generalised linear mixed models (GLMMs) with Log link function in which individual eagle territory was included as a random effect and success (1)/failure (0) as the response variable. Our predictors were region, year or density classes, respectively. To assess the goodness of fit of our set of prediction models, we used scatter plots of residual diagnostics. Due to the homogeneously dispersed variance and lack of good goodness-of-fit of the models, GLMMs were dropped, and analyses were performed using simple GRMs. In our set of the GRMs, again the tested breeding parameter (OR, P, BS, BF) was a response variable, and factors were region, year or density.

We collected data on the breeding distribution of the LSE in our study area in the period 2015–2024. The study area was divided into 2 × 2 km grids. We visited each grid at least once in the period April–July, with observations carried out in favourable weather conditions from observation points of good visibility, lasting 2–3 h. In general, there was a single observation point in each grid, except for those marked by a highly rugged terrain, where two to three observation points were identified. For more details, see [16]. We calculated the breeding density of the LSE after accounting for the number of registered occupied breeding territories in an area of 100 sq. km. for each of the studied regions.

To assess the impact of density, we used a modified and relative to our reality variant of breeding density classes in LSE [23]. Thus, we adopted the following nest density classes for the species: low (<3 pairs/100 km²), medium (3.1 to 7 pairs/100 km²), and high (>7.1 pairs/100 km²).

In the analysis, we included only territories that were monitored for at least two consecutive years to exclude stochastic events.

Explanatory parameter estimates (β²) with lower (95%) and upper CL (95%) and a probability value (p) of the explanatory factors were evaluated. Results with p ≤ 0.05 were considered significant. Values were provided as means ± standard error (SE). Statistica for Windows, Release 12 [24] was used for the statistical analysis of the data.

3. Results

3.1. Overall Breeding Parameters

Over the ten-year study period, the mean occupancy rate (±SD) of the territories (n = 427) was 0.98 ± 0.03, demonstrating a marginal decrease β² = −0.64 ± 0.27, F_1.8 = 5.49, p = 0.047 (Figure 2). This process was clearly visible in Sakar Mnt. (β² = −0.66 ± 0.27, F_1.8 = 6.19, p = 0.038), where in 2022, only 67% of the monitored territories were occupied. The overall mean productivity (±SD) was 0.50 ± 0.08, mean breeding success (±SD), 0.75 ± 0.14, and the mean breeding frequency (±SD), 0.67 ± 0.09 (

Table 2. Comparison of the breeding performance of the Lesser Spotted Eagle in different parts of the distribution.

Country	Breeding Parameter				Period	References
	Productivity (Fledglings/Occupied Territories)	Breeding Success (Fledglings/Incubated Pairs)	Breeding Frequency (Incubated Pairs/Occupied Territories)	Occupancy Rate (Occupied Territories/Monitored Territories)
Estonia	0.62				1981–2002	[25]
Estonia	0.56	0.78	0.69		1992–2009	[4]
Estonia	0.69				1999–2002	[5]
Estonia	0.44				2002–2010	[10]
Estonia	0.67				2004–2006 2010–2012	[26]
Latvia	0.58	0.79	0.68		1985–1996	[13]
Latvia			0.58		1988–2003	[8]
Latvia	0.49	0.74	0.66		1988–2014	[23]
Latvia	0.43				2002–2010	[10]
Latvia				0.83	2006–2017	[27]
Lithuania	0.61				2001–2003	[12]
Lithuania	0.60				2001–2006	[28]
Lithuania	0.65				2002–2010	[10]
Lithuania				0.90	2012–2017	[29]
Belarus	0.76				1981–1991	[30]
Poland	0.63				1988–1991	[31]
Poland	0.69				??	[32]
North-Eastern Poland	0.54			0.81	1999–2015	[33]
Eastern Germany	0.51		0.78		1994–1997	[34]
Eastern Germany	0.65				??	[35]
Slovakia	0.53		0.77		1966–1978	[36]
Slovakia	0.51	0.69	0.75		2011–2014	[37]
Hungary	0.68				??	[38]
Eastern Georgia/Western Azerbaijan	0.94	1.06			1982–1992	[39]
Eastern Georgia	0.98	1.09			1982–1988	[40]
Northern Caucasus, Russia	0.71				2007	[41]
Northern Greece	0.67				1985–1987	[42]
South-Eastern Bulgaria	0.50	0.67	0.74		2014–2018	[16]
Bulgaria	0.50	0.75	0.67	0.98	2015–2024	This study

). However, none of these three parameters indicated any trend over the years (p > 0.05) (Figure 2).

3.2. Temporal Patterns of Breeding Performance

We found that the occupancy rate differed significantly over the years β² = 0.29 ± 0.14, p = 0.04, being lower in 2021 (0.93 ± 0.03). In the period 2015–2019, all verified eagle territories were occupied, followed by a decline in the last five years (Figure 2). Breeding success also demonstrated significant temporal variability β² = −0.12 ± 0.06, p = 0.04. Its value was lower in 2019 (0.52 ± 0.11, n = 23 breeding attempts), followed by the highest value recorded the next year, BS = 1 (n = 20 breeding attempts) (Figure 2). However, the other two analyzed breeding parameters did not show significant differences over the years (

Table 3. Results of sigma-restricted parametrized simple general regression models (GRMs) constructed to analyze the effect of different factors on the breeding performance of the Lesser Spotted Eagle. Explanatory parameter estimates β² ± Std. Err. with lower (95%) and upper CL (95%), and a probability value (p) with degree of freedom (df) of the explanatory factors are given. Significant values are indicated in bold.

Parameter	Factor Effect	β2	Std. Err.	Lower CL	Upper CL	df	p
Occupancy rate	Year	0.29	0.14	0.02	0.55	1.415	0.04
Productivity	Year	−0.06	0.04	−0.14	0.02	1.337	0.12
Breeding success	Year	−0.12	0.06	−0.23	−0.003	1.226	0.04
Breeding frequency	Year	−0.01	0.04	−0.09	0.08	1.337	0.86
Occupancy rate	Region	0.19	0.11	−0.02	0.41	1.425	0.08
Productivity	Region	0.02	0.04	−0.07	0.10	1.346	0.68
Breeding success	Region	0.09	0.06	−0.02	0.20	1.235	0.12
Breeding frequency	Region	−0.05	0.05	−0.15	0.04	1.346	0.27
Occupancy rate	Density	−0.75	0.39	−1.51	0.01	1.424	0.054
Productivity	Density	0.25	0.12	0.02	0.48	1.345	0.03
Breeding success	Density	0.15	0.16	−0.16	0.47	1.234	0.34
Breeding frequency	Density	0.27	0.12	0.02	0.51	1.345	0.03

).

3.3. Regional Patterns of Breeding Performance

None of the studied breeding parameters manifested significant spatial differences (Table 3). Yet, the lowest mean value of occupancy rate (0.91 ± 0.04) was recorded in Sakar Mnt. (

Table 4. Mean value with std. err. of different breeding parameters of the Lesser Spotted Eagle population in different studied regions in Bulgaria.

Region	N	Occupancy Rate	Std. Err.	No.	Productivity	Std. Err.	No.	Breeding Success	Std. Err.	No.	Breeding Frequency	Std. Err.
Suha Reka river	31	0.97	0.03	25	0.76	0.09	21	0.90	0.07	25	0.84	0.07
Strandzha Mnt.	25	1.00		25	0.48	0.10	19	0.63	0.11	25	0.76	0.09
Dervent Heights	199	0.99	0.01	164	0.46	0.04	101	0.74	0.04	164	0.62	0.04
Rusenski Lom	22	1.00		19	0.58	0.12	11	1.00		19	0.58	0.12
Batova river	7	1.00		7	1.00	0.22	7	1.00	0.22	7	1.00
Sakar Mnt.	66	0.91	0.04	48	0.46	0.07	30	0.73	0.08	48	0.63	0.07
Eastern Balkan	25	1.00		21	0.62	0.11	18	0.72	0.11	21	0.86	0.08
Gorata ridge	9	1.00		7	0.71	0.18	6	0.83	0.17	7	0.86	0.14
Byala reka river	20	0.95	0.05	15	0.33	0.13	12	0.42	0.15	15	0.80	0.11
Tundzha river	23	0.96	0.04	17	0.47	0.12	12	0.67	0.14	17	0.71	0.11
TOTAL	427			348			237			348

). It should be noted that the Byala reka area recorded the lowest mean productivity (0.33 ± 0.13) and the lowest mean breeding success (0.42 ± 0.15). The lowest value of mean breeding frequency (0.58 ± 0.12) was observed in the Rusenski Lom region, but high breeding success and occupancy rate were found here (Table 4). The highest breeding performance was found in the Batova River area, but caution is required on these results, because of the small sample size (Table 4). However, a high value of breeding parameters was also recorded in the Suha Reka region, where mean productivity was 0.76 ± 0.09, mean breeding success, 0.90 ± 0.07, and mean breeding frequency, 0.84 ± 0.07. We found that a lower value of mean breeding frequency was also observed in the Dervent Heights region (0.62 ± 0.04) and the neighboring region of Sakar Mnt. (0.63 ± 0.07).

3.4. Breeding Performance vs. Breeding Density

As we expected, breeding density affected most of the tested breeding parameters (Table 3). However, the occupancy of territories was marginally influenced by eagle’s density (β² = −0.75 ± 0.39, p = 0.054), so that in places of high density, occupancy was higher (0.99 ± 0.01) than in low (0.98 ± 0.02) or middle classes (0.96 ± 0.02). We found that eagle’s productivity was influenced by density, β² = 0.25 ± 0.12, p = 0.03. In contrast to our expectation, LSE found in high-density conditions (n = 164 breeding attempts) had nearly the same mean productivity (0.46 ± 0.04) as that (0.43 ± 0.08) recorded in a low-density environment (n = 40 breeding attempts). Yet, eagles reproducing in middle density (n = 144 breeding attempts) had the highest mean productivity (0.59 ± 0.04). The same phenomenon was also observed in breeding success, although density did not significantly affect the reproductive outcome (Table 3). Regarding our assumption, breeding frequency was strongly impacted by density, β² = 0.27 ± 0.12, p = 0.03, in a way that in high-density areas (n = 164 cases) eagles bred less frequently (62% ± 4), than those in middle (73% ± 7) (n = 144) or low-density conditions (78% ± 4) (n = 40).

4. Discussion

4.1. Breeding Parameters: Variation in Space and Time

Although the different aspects of demography of the LSE are relatively well studied in a number of countries where the species occurs [4,9,10,26,43], little is known about the occupation rate of the territories [27,29]. However, for raptors, occupancy is a measure of territory quality [21,22], since high-quality territories were more often occupied than low-quality ones according to the habitat heterogeneity hypothesis (HHH) [44,45]. Although our mean value of territory occupation was higher than that reported in Latvia, Lithuania, and North-Eastern Poland (Table 2), of the tested demographic parameters, only the occupancy rate demonstrated a significant decrease, a process particularly noticeable in Sakar Mnt. Interestingly, between 2015 and 2019, all of the monitored LSE territories were occupied, followed by a decrease in occupation in the next five years, as the lowest mean value (0.93) was recorded in 2021 (Figure 2). Although no significant variation was recorded in a regional context (Table 3), a lower occupancy rate was observed in the south-eastern regions such as Sakar Mnt., the Byala Reka River, and the Tundzha River valley (Table 4). For the Byala Reka River and the Tundzha River valley, these results might be explained by the relatively low sample sizes.

We hypothesize that the decrease in territory occupancy in Sakar Mnt. is determined by different drivers. First, the territory selection of LSE in south-east Bulgaria was determined by the preferences for grasslands and agricultural mosaic as well as the avoidance of extensive arable land. Habitat preferences were related to high habitat diversity in the home ranges, as the eagle selected large fragments of grassland or grassland with dispersed patches of shrubs [17]. However, in the last two decades, the land-use pattern in south-east Bulgaria was subject to a significant transformation expressed as widespread conversion of grasslands into cropland and establishment of large arable monoculture fields mainly sown with cereals, sunflower, and oil rape [21]. Indeed, spectacular loss of different types of grasslands, especially permanent pastures, was recorded in Sakar Mnt., while arable lands increased significantly due to the plowing of grassland biotopes. Furthermore, the process of devastation of grassland habitats with shrubs consisted of removing shrub vegetation, which was either burnt down or destroyed by shredders and bulldozers. It was found that landscape alteration affected the demography of another large top predator, such as the Eastern Imperial Eagle (Aquila heliaca), that exploited the same foraging pattern. Habitat transformation had a significant negative impact on the occupancy rate of Eastern Imperial Eagle territories by reducing their habitat quality, while productivity and breeding frequency showed no trend [21]—a process very similar to the scenario occurring with LSE, a species utilizing similar foraging habitats and probably suffering similar impacts of land-use pattern change.

Another possible explanation of the declining occupancy rate of LSE breeding in Sakar Mnt. is the devastating forest fires, especially intensive in the last five years, spreading over hundreds of hectares and destroying both the species’ breeding and feeding habitats (authors’ data). This is a plausible explanation, especially in the context of climate change, dramatically affecting the Sakar Mnt. region and resulting in drought, drying up of rivers and water bodies (another element of the landscape to which LSE is selective, see [17]. desertification of habitats, and intense fires.

And last but not least, the relationship of LSE with other top predators, such as the Eastern Imperial Eagle and White-tailed Eagle (Haliaeetus albicilla), is gradually increasing as breeders in the region [46]. No adverse effect on LSE caused by White-tailed Eagle was found in the Baltic region [29], but it was proven that LSE was subject to predation by the Eastern Imperial Eagle [47] or Eagle Owl (Bubo bubo) [48]. There were also observations on the displacement of LSE territories by the Eastern Imperial Eagle and Eagle Owl (authors’ data). However, in our study system, on the one hand, LSE was in the role of a mesopredator, dominated by large top predators such as Eastern Imperial Eagle, White-tailed Eagle, or Eagle Owl, and, on the other hand, it was facing serious habitat transformations driven by human activities and climate change. Further research to clarify the impact of all these factors, both separately and cumulatively, would be particularly useful.

Breeding performance of raptors is influenced by different factors such as habitat, food supply, disturbance, weather conditions, and competition [1,3,4,49]. Reproductive outcome of LSE differed significantly, both temporally and spatially, over large geographical areas [8,9,10], with no significant trend [4,23]. It was found that food abundance [4,11,12], weather conditions [4,13,33], or land use pattern [9,50] strongly influenced the breeding performance of this forest-dwelling raptor species. The lack of a significant trend in the main breeding parameters confirmed the consistent reproduction of LSE in the southern periphery of its range, as it was recorded along the northern limit [4] or in the core area of the species’ distribution [23]. However, our average productivity (fledgling per occupied territories) was lower than reported for many countries such as Estonia, Lithuania, Belarus, and Poland, but similar to the value published for Latvia, Eastern Germany, and Slovakia (Table 2). It was far from the high output found in the Caucasus region (Table 2). This corresponded to the “abundant-center” hypothesis [51] stating that the population situated at the limit of the distribution would have the poorest rates of reproduction, because better conditions for the species were expected to occur in the center of the range rather than in the periphery [52]. However, in different parts of the species’ range, reproductive output was driven by different factors over the years. It was found that spring temperature and vole abundance determined the productivity of the species in the northern limit of the distribution [4]. The productivity of LSE was positively correlated with the share of small rodents in the diet [12], without any adverse effect caused by large competitors [29]. Broods with two fledglings had been reported regularly from the whole species’ range, varying from 1.6% in Latvia [23] to 3.4% in Belarus [30]. In our study, we had only one successful brood with two fledglings, i.e., 0.54%. The survival of the second nestling in a brood could be the result of the lowered aggression between siblings in food-rich conditions [4], corresponding to the center of the range, where better conditions for the species are expected to occur [52]. Indeed, further research investigating the relationship between productivity and different factors affecting reproduction, such as diet, weather, habitat, and presence of intra- and interspecific competitors in the southern periphery of the LSE distribution, would be especially valuable.

The same value of average productivity was found by us in the previous study published for the southern part of the country [16], which supported our results concerning the consistent reproduction of LSE in this part of the distribution.

We found that the mean breeding success (fledglings per incubating pair) of LSE from Bulgaria fluctuated significantly over the years, but not between the regions in the country. Its value was similar to that in other parts of Europe, yet with a slightly higher value than that reported for south-eastern Bulgaria in the previous study period (Table 2). However, this difference was mostly due to the larger sample size in the present study, including regions in the northern part of the country, where birds had higher breeding success. The high value of the breeding parameters observed in Suha reka, the north easternmost region in the study area, was probably due to the abundance of prey and good breeding conditions, as well as the lack of other large competitors. Yet, this assumption needs further verification.

Regarding the average ratio of birds that had laid eggs and started breeding to those that had just occupied territory (breeding frequency), Bulgaria was close to Estonia and Latvia, but had a lower average value than that reported in East Germany and Slovakia [34,36,37] (Table 2). Breeding frequency was positively related to warm pre-laying period (temperature in April) and wet preceding season in Estonia [4]. According to (35: 288), birds living at lower latitudes depend mostly on rainfall, while at high latitudes, the most important weather factor is temperature. Warm weather and prey abundance during the pre-laying period probably induce the onset of egg-laying. These factors may affect different aspects of the state of birds, from successful foraging to the attainment of the physical condition and hormonal level necessary for breeding [53] and vice versa, bad weather in the pre-breeding phase and low prey abundance may force birds to desist from breeding or even not occupy the territory [54]. These mechanisms may influence the breeding frequency of LSE [4]. We speculate that dry and unstable weather, expressed in frequent and sharp temperature downfalls in April, caused by the effect of climate change, especially clearly affecting the geographical latitude of Bulgaria, was the reason for the low breeding frequency of the species in some years. However, this issue needs further clarification.

Compared to the previous study, which focused on the south-eastern part of the country, a difference in average breeding frequency was also observed, which could be explained by the different sample size and the inclusion of new regions in the study.

As it was found in the previous study [16], the lowest reproductive output was recorded for the Byala Reka region, probably related to the habitat availability in the home ranges. The LSE tends to forage in different types of grasslands, avoiding arable lands and large forests [9,10,33]. The home ranges of the species in the Byala Reka region consisted mainly of large forest patches and arable lands of different sizes, including a lower percentage of grasslands. Thus, in these territories, the eagles most likely persisted in suboptimal habitats, where in poor-food years bred unsuccessfully or did not reproduce [55]. Comparing our results with the previous study conducted in the south-eastern part of the country [16], we see a similarity in the values of the studied breeding indicators for the vast areas of Sakar Mnt. and the neighboring Dervent Heights, which is another confirmation of the consistency of the species’ reproduction. The differences reported between the two studies for the areas of Strandzha Mnt. and the Tundzha River Valley are due to the small sample size for these regions in the first study, leading to a bias.

4.2. Breeding Parameters and Effect of Density

The high occupancy rate of territories situated in regions of high density could be explained by the high quality of these territories according to the habitat heterogeneity hypothesis. Contrary to our presumption, LSE reproducing in high density had the same, even slightly higher average productivity, compared to eagles breeding in low density. But why does such contradiction arise? We propose the following explanation.

According to the habitat heterogeneity hypothesis (HHH), habitats are of varying quality. Ideal despotic distribution (IDD) theory [56] predicts how individuals of varying competitive abilities distribute themselves in a habitat of differing resource qualities. Unlike the ideal free distribution (IFD) [57], which assumes equal fitness for all individuals, IDD states that dominant individuals will occupy the best territories or areas with the most resources, while subordinate individuals will be forced into less desirable areas with fewer resources. Dominant individuals may require smaller territories in high-quality areas compared to subordinates in lower-quality areas, as the quality of the territory offsets the need for a larger area. The average productivity in good sites is expected to be equal in both low- and high-density situations [55]. Therefore, the quality of the territories could explain the almost equal productivity of eagles breeding in both high-density and low-density conditions. It is a trade-off situation in which eagles breeding in high-density conditions suffer from the negative effects of increased competition for resources, but occupy high-quality habitats, while in the other case, they stay in a sub-optimal landscape, but have larger territories and are slightly affected by competition for resources.

The strong correlation between breeding frequency and breeding density found by us supposes regulation of reproduction frequency by density. However, in areas of high density, at the beginning of the breeding season, eagles occupying a given territory are forced to constantly defend it from intruders of the same species or from other predators, which involves high energy expenditure. In some years, poor weather conditions, sometimes combined with poor prey abundance, may be the reason for a low breeding frequency, as eagles mainly focus on holding the territory. This is evidenced by the observations of birds that, after returning from a long migration, occupy one of their nests and decorate it with green branches, but do not proceed to breeding. Maintaining the “fresh” nest can continue throughout the breeding season, and there are known cases when they even decorate and maintain more than one nest, but in the end, do not breed anyway (authors’ data). Conversely, in areas of low density, eagles face little competition, and even if the territories are of lower quality, especially in “good years”, the birds proceed to breeding. However, identifying the effect of other factors that influence breeding density, such as weather and prey abundance, is crucial for understanding all mechanisms that determine when eagles start breeding or just occupy territory.

4.3. Conservation Recommendation

Recently, the land-use pattern in south-east Bulgaria was subject to spectacular habitat transformation, as this was notably harmful in Sakar Mnt. At the same time, the LSE was faced with a novel threat, that of climate change, such as devastating fires, severe drought, and aridisation, which is apparently occurring much faster and more severely than predicted scenarios. Therefore, all these circumstances draw an uncertain future for LSE along the southern limit of its distribution, especially in the regions of Sakar Mnt., neighbouring Dervent Heights and Eastern Balkan Mnts. Prompt actions and urgent, yet informed decisions are needed to prevent the negative consequences of these imminent threats. Based on our research, we propose several important solutions to be taken into account by responsible institutions and organizations.

First of all, improving the forest fire early warning and rapid response system, is of key importance. This also involves the purchase of new and sufficient equipment, including aerial extinguishers (specialized airplanes and helicopters). The construction and maintenance of a network of small reservoirs (up to 100 sq. m) is also of fundamental importance, since, on the one hand, they will provide water for extinguishing the fire, and, on the other hand, they are of key importance as a habitat supporting the abundance of amphibians, an important nutritional component in the eagle’s diet [12]. It has been proven that the availability of such water bodies is of crucial significance for territory selection by LSE in these latitudes [17].

Forest transformation, replacing tree species (Pinus, Cedrus, and Abies), atypical of the region and altitude, planted more than 60–70 years ago on vast territories throughout eastern Bulgaria, is one of the main policies to be implemented in a very short time. Indeed, the replacement of tree species uncharacteristic of these places with typical deciduous species, such as Downy Oak (Quercus pubescens) and Hungarian Oak (Quercus frainetto), resistant to forest fires, has already begun, but the process needs to be intensified.

Conservation efforts should also be focused on the restoration of habitat complexity, which includes the recovery of plowed grasslands and the establishment of mosaic habitat structures, consisting of complexes of permanent grasslands with scattered small shrub formations and grassland margins between medium-sized arable patches. The preservation of the species’ main foraging habitats and the restoration of damaged ones will enhance the connectivity of preferred habitats and thus benefit the good conditions of prey abundance and availability. Our recommendations should be implemented in the feature forestry management plans to ensure their proper application and sustainability.

Finally, further research to investigate the relationship between the breeding performance of LSE and different factors affecting its reproduction, such as diet, weather, habitat features, and the presence of intra- and interspecific competitors, would be particularly valuable and would bring insight into the dependencies of this species at the southern limit of its distribution. Studying the effects of fires in the context of climate change and predicting the response of eagles as key indicators under different scenarios is of particular importance for acquiring knowledge about the functional state of the entire ecosystem in a rapidly changing environment. Only then will the knowledge gained ensure that the right decisions are made and prompt actions taken to conserve this iconic species.

5. Conclusions

In our work, we presented data on the LSE main breeding parameters in the country, collected over a 10-year period, comparing them with other parts of the species’ range. We found that the mean occupancy rate of the territories decreased, as this process was more visible in Sakar Mnt. The overall mean productivity, breeding success, and breeding frequency showed no significant trend in the study period, confirming the consistent reproduction of LSE in the southern periphery of the range, corresponding to findings in the north [4] or center of the species’ distribution [23].

Of the tested breeding parameters, occupancy rate and breeding success varied yearly, while productivity and breeding frequency were influenced by density. None of the tested indicators demonstrated significant regional differences, which reveals a temporally determinate, but spatially consistent pattern of breeding performance of the species in this part of the distribution.

The assumption that population regulation was influenced by breeding density found partial confirmation. We established that the reproductive outcome was not constrained by breeding density (high-density and low-density areas had similar productivity), but most likely by territory quality. Then, reproduction was influenced by territoriality as found in the Baltic region [10]. On the other hand, density negatively correlated with breeding frequency, so a density-dependent mechanism most probably worked with regard to breeding, suppressing the initiation of nesting by eagles in high-density regions, especially in poor years.