Comparison on the Prey Remains of Reintroduced Lesser Kestrels (Falco naumanni Fleischer, 1818) Colonies in Bulgaria

Abstract

This study presents preliminary data on the diet composition of reintroduced Lesser Kestrels (Falco naumanni) in two restored colonies in Bulgaria—near Stara Zagora and in the Sakar Mountains at Levka village—based on pellet analysis. A total of 103 and 48 identifiable prey items (occurrences in the pellets) were recovered from the respective sites. Insects (particularly Coleoptera) dominated the diet at both colonies, but some differences were observed in the occurrence of vertebrate prey. Birds (Aves) were the second most common category in Stara Zagora, while small mammals (Mammalia) were more prevalent in Sakar. Annelids were detected only in Stara Zagora, and millipedes (Diplopoda) only in Sakar. Synthetic materials were found in pellets from Stara Zagora, suggesting plastic ingestion. Levins’ index indicated similar niche breadths (1.915 vs. 2.008), and Pianka’s index showed a high dietary overlap (0.981), pointing to broadly similar trophic preferences despite local prey availability.

1. Introduction

The Lesser Kestrel (Falco naumanni Fleischer, 1818) is a species that feeds primarily on invertebrates, but its diet has not been studied in detail in Bulgaria in the past [1,2]. In the past, the species was widespread throughout Bulgaria [3], following a drastic decline in its numbers over the last century, and even disappeared as a breeding species from the country [4]. Data on the prey of this species from the last century in the Eastern Rhodopes mention representatives of the order Orthoptera and the families Tettigonidae, Acrididae, Gryllidae, Carabidae, Tenebrionidae, etc., as well as some lizards (Lacerta sp.) [2,5]. In more recent times, insects and small reptiles have been briefly described as part of the species’ diet. [6].

After the species’ nesting was restored successfully in the country in 2014 [7], to date, the species’ diet has only been examined in the Sakar Mountains, where 54 pellets were collected from beneath nest boxes in May 2022 [8]. Some data on the prey of Lesser Kestrel in the area of Sakar are also included in the papers of other authors [3,9,10]. These data confirm that, as in other European and neighboring countries [11,12,13,14], in Bulgaria, the species is a pronounced insectivorous bird, with some data showing the presence of over 90% insects in its food items. These are mainly representatives of Orthoptera, Scolopendra sp., Cicadidae, and Coleoptera, while some species of rodents (Muridae and Arvicolinae) and reptiles, such as Lacerta viridis, occur in the diet of the Lesser Kestrel at much lower proportions—around 4% and 2%, respectively [9]. Representatives of Coleoptera (60.5%), Hymenoptera (17.65%), Orthoptera (5.88%), as well as Scolopendromorpha (3.36%) are also reported as the most frequently recorded prey for the first reintroduced colony of the species in the country, while Rodentia make up only 8.4% of the recovered food remains [8]. Other studies have examined the possibility of using photographs and video footage as a method to research the diversity of prey consumed by Lesser Kestrels in different colonies in Türkiye and Bulgaria [15]. In neighbouring Greece, its diet includes various insects (mainly Coleoptera and Orthoptera), as well as some centipedes (Chilopoda) [16,17]. Reptiles and small mammals are consumed much more rarely. Similar data are reported for another country neighboring Bulgaria—Türkiye—where Orthoptera and Coleoptera are also the most frequently found in food remains [18]. The observed similarities in the diet of the species in the three neighboring countries, along with the wide distribution of insects such as Orthoptera and Coleoptera, provide grounds to assume that the availability of food resources for the Lesser Kestrel in Bulgaria is sufficient, and in this regard, future reintroduction projects in new areas of the country may be successful.

In this study, for the first time in the country, we compare the diet of the Lesser Kestrels from two different breeding colonies, where they were recently reintroduced by Green Balkans under the implementation of two different projects under the LIFE Programme of the European Union [19].

2. Materials and Methods

Pellets and food remains were collected in two separate, distant colonies, which cover over 70% of the breeding habitats of the species in Bulgaria, following its restoration in the country in 2014 (Figure 1).

One of the studied colonies is included in the scope of SPA Sakar (BG0002021), part of the NATURA 2000—common European ecological network in the country [20]. This colony is located in the vicinity of Levka village in the Sakar Mountains (41°52′17.1″ N 26°15′30.1″ E; 243 m. above sea level), and this is the first place where the species has been recovered as a breeder in the country [7]. The area of Sakar Mts. is used mainly for agriculture, livestock, and forestry. This is a low-mountain region with rounded hilltops and comparatively open river valleys of the Maritsa and Tundzha tributaries, close to the state border with Türkiye. The altitude of the terrain is between 50 and 856 meters above sea level [20]. The breeding and foraging habitats of the Lesser Kestrels here are represented by the following Ecosystem types: Cropland, Grassland, and Urban [8].

The second colony of Lesser Kestrel, which is the subject of our study, is located in the vicinity of the city of Stara Zagora in the yard of the Wildlife Rehabilitation and Breeding Center (WRBC) of Green Balkans (42°24′16.9″ N 25°38′08.9″ E, 175 m. above sea level). This is the second place in the country where the species has been purposefully reintroduced by Green Balkans; thus, the breeding distribution of the species in the country ranges from 8 to 243 m above sea level [19]. The city of Stara Zagora, located on the border between the Upper Thracian Lowland and the Sarnena Sredna Gora Mts. The region includes extensively cultivated agricultural areas, urban environments, and industrial sites, and mainly covers cropland and urban ecosystem types, assessed by MAES [21].

Pellets, N = 76, and a small amount of food remains were collected on 14 May and on 3 June 2025 from the yard of WRBC, and N = 36 during February and April 2022 and 2023 from the village of Levka, Sakar Mts. They were stored in plastic bags and later identified in the laboratory. Pellets were collected from beneath communal nesting and roosting sites within the colonies. As it was not possible to distinguish pellets from individual birds, the collected samples represent a collective dietary snapshot of the entire colony rather than individual diets.

To evaluate the dietary niche breadth and the degree of trophic overlap between the two reintroduced Lesser Kestrel colonies (Stara Zagora and Sakar), we used quantitative indices based on prey item frequencies recovered from pellets (occurrences). Niche breadth was calculated using Levins’ index [22], and dietary overlap between the two colonies was estimated using Pianka’s index [23]. All prey items were grouped into broad taxonomic categories to standardize comparison. No biological fragments from the pellets remained unidentified.

It is important to acknowledge the limitations inherent in using pellet analysis to determine diet composition. This method may underestimate the consumption of soft-bodied prey (e.g., Annelida, larvae) that leave few indigestible remains. Furthermore, our results represent a snapshot of the diet during the specific collection periods and may not reflect long-term or seasonal variation.

3. Results and Discussion

3.1. Colony at Stara Zagora

A total of 103 identifiable prey items and other remains were recovered from the analyzed pellets of reintroduced Lesser Kestrels (Falco naumanni) near the city of Stara Zagora, Bulgaria (

Table 1. Occurrences and percentiles of different indigestible prey remains in the pellets of Lesser Kestrels from the Sakar Mts and Stara Zagora.

Prey Item	Sakar		Stara Zagora
	N	%	N	%
Annelida	0	0.00	3	2.97
Insecta	33	64.71	77	76.24
Diplopoda	1	1.96	0	0.00
Aves	3	5.88	13	12.87
Mammalia	14	27.45	8	7.92
Total	51	100.00	101	100.00

). This is one of the restored colonies of the species in Bulgaria, as part of the activities of Green Balkans [19]. The most frequently represented group was beetles (Coleoptera), accounting for 66% of all records (n = 68). Remains of birds (Aves) were the second most common category, representing 12.6% (n = 13), followed by mammals (Mammalia) with 7.8% (n = 8), and orthopterans (Orthoptera) with 5.8% (n = 6). Less frequently detected prey included Hymenoptera and Annelida, each comprising 2.9% (n = 3) of the records. In addition, fragments of synthetic materials were found in two pellets (1.9%), indicating occasional ingestion or incorporation of plastic remains (Figure 2). There are no other studies in the country that identify Aves taxa as the second most important component of the diet of this species of falcon, although Passer sp. (pulls) have already been described as part of the diet of Lesser Kestrels in Bulgaria, they account for only a few recorded cases [8]. Nevertheless, it should be noted that breeding of the species in the country was restored only 10 years ago, and current data on the trophic ecology of the Lesser Kestrel are still scarce and should be the subject of future studies. Pellet analyses in other countries also indicate birds as rarely captured prey [24,25]. The adaptation of Lesser Kestrels from this colony to capturing more birds could be speculated to result from the colony’s location in an urbanized area, where Passeriformes are widespread and abundant.

The identified prey items included representatives of six major groups: insects, annelids, mammals, and birds, along with non-food synthetic material.

The most diverse group included the insects (Insecta), with multiple representatives from the orders Coleoptera (including families Carabidae, Dytiscidae, Scarabaeidae (with species from the genera Melolontha and Cetonia), and Orthoptera (Tettigonia sp.). Additionally, remains of Hymenoptera (Formicidae) were recorded.

Vertebrate animal remains were also present in the samples. These included small mammals, such as the common vole (Microtus arvalis complex), and birds, primarily Passeriformes, represented in the form of feathers and bones. The presence of Mammalia and Aves in the diet suggests opportunistic feeding on small vertebrates.

Annelid remains were also detected, particularly from the family Lumbricidae. Annelids are likely consumed opportunistically, particularly after rain, when they are more accessible on the surface. Furthermore, synthetic fragments were found in several samples, likely of plastic origin, suggesting incidental ingestion of anthropogenic material. This is the first time such data have been reported for the food remains of Lesser Kestrels in the country, and the presence of plastic fragments is likely related to the colony’s proximity to a large city, which is a source of large amounts of anthropogenic waste. Similar data on the presence of plastic waste in the pellets have been reported for the White Stork (Ciconia ciconia) [26]. Similar to the Lesser Kestrel, which is described as a characteristic farmland bird [27], the White Stork also belongs to the group of farmland birds [28]. This determines the shared foraging habitats of the two species, with pellets from the White Stork also containing dominant remains of similar invertebrate prey taxa (especially from the orders Coleoptera, Orthoptera, and Hymenoptera) [26]. Plastic waste (8.4%) and cigarette filters (6.9%) are the most widespread anthropogenic wastes, found in 22.7% of pellets or 34.2% of the breeding pairs of White Storks examined in this study. The ingestion of plastic and other indigestible anthropogenic litter can be very dangerous to bird health and may lead to death. At the Green Balkans—Stara Zagora NGO—Wildlife Rehabilitation and Breeding Center, a number of cases have already been recorded of White Storks ingesting various agricultural, veterinary–medical, and household wastes, such as plastic elastic bands, which, once entering the digestive system, can cause blockages, intoxication, or other lethal effects. Regarding the Lesser Kestrel, there are still no confirmed similar cases; however, given that the first plastic fragments have been reported in the food remains of Lesser Kestrels, more in-depth and detailed research on this topic is necessary in the future.

3.2. Colony at Sakar Mts

A total of 48 identifiable prey items and other remains were recovered from the analyzed pellets of reintroduced Lesser Kestrels (Falco naumanni) in the Sakar Mountains, Bulgaria. This site represents another restored colony of the species, developed through the conservation efforts of Green Balkans.

The most frequently represented group included beetles (Coleoptera), accounting for 35.4% of all records (n = 17). Mammals (Mammalia) followed closely with 29.2% (n = 14), while unidentified insect remains (Insecta indet.) constituted 27.1% (n = 13). Less frequently recorded prey included earwigs (Dermaptera) with 4.2% (n = 2), and orthopterans (Orthoptera) and millipedes (Diplopoda), each with 2.1% (n = 1). The detection of millipedes, which are not a common prey for raptors, suggests highly opportunistic or perhaps incidental feeding behaviour, possibly during periods when other preferred prey are less abundant.

The identified prey items included representatives of insects, mammals, and myriapods. Insects (Insecta) were the most diverse group, dominated by beetles (Coleoptera: Carabidae, Tenebrionidae, Scarabaeidae—Cetonia sp.), followed by unidentified Dermaptera and Orthoptera. Numerous fragments of insects were not identifiable beyond class level (Insecta indet.), but they still represented a substantial portion of the diet.

Small vertebrate remains of mammals (M. arvalis complex and Mus musculus) were present in nearly one-third of the analyzed pellets, suggesting frequent opportunistic feeding on small mammals. This pattern differs from the Stara Zagora colony, where birds (Aves) were more prevalent.

The presence of Diplopoda is noteworthy, as these detritivores are not a common prey for raptors, indicating a highly opportunistic or perhaps incidental feeding behavior. No synthetic or plastic materials were detected in this set of pellets, unlike in the Stara Zagora colony.

A comparative summary of the major prey groups of Lesser Kestrels in Sakar and Stara Zagora reveals some differences. In both colonies, insects (Insecta) formed the bulk of the diet, with a higher number recorded in Stara Zagora compared to Sakar. Vertebrate prey also differed: birds were more frequently detected in Stara Zagora than in Sakar, while mammalian remains were more common in Sakar than in Stara Zagora. Annelid remains were present only in the Stara Zagora samples, whereas millipedes were recorded exclusively in the Sakar colony.

Niche breadth was calculated using Levins’ index [22], resulting in a value of 1.915 for the Stara Zagora colony and 2.008 for the Sakar colony. These values suggest a relatively similar degree of dietary specialization in both colonies, with slightly broader niche breadth observed in Sakar. The high dietary overlap (Pianka’s index = 0.981) between the two colonies, despite their different habitats, indicates a strong similarity in the fundamental trophic preferences of the Lesser Kestrels in these restored populations. This suggests that the species maintains a consistent dietary niche centred on insects, particularly Coleoptera, even when local prey communities show some variation. The minor differences observed in vertebrate prey consumption (Aves vs. Mammalia) likely reflect local prey availability and opportunistic hunting, rather than a fundamental divergence in feeding strategy.

To statistically compare the dietary composition between the two colonies, a chi-square test was performed on the frequencies of the main prey groups. The results indicated a significant difference in the proportions of prey items between the Stara Zagora and Sakar colonies (χ² = 14.57, df = 4, p ≈ 0.0056). We acknowledge that some expected frequencies were below 5, which is a limitation of this test. Nevertheless, the results provide strong evidence for a significant difference in the realized diet of the Lesser Kestrels between the two study sites.

This study focused on the realized diet of the Lesser Kestrel as determined by pellet analysis. We acknowledge that a comprehensive understanding of the species’ trophic ecology would require data on both the realized diet (from pellets) and the potential diet (prey availability in the habitat). The current study provides a foundation by characterizing the actual diet, and we propose that future research should integrate assessments of prey availability to better inform species management and reintroduction strategies. While we did not calculate prey biomass, our frequency-based approach is a common and valid method in dietary studies. However, we recognize that including biomass estimates could provide a more complete picture of the energetic contribution of different prey groups.