What the Owls Leave Behind: Pellet Size Variation Reflects Predator Body Size in Israel’s Owls

Abstract

Owl pellets offer a distinctive, noninvasive perspective on the feeding ecology and morphological traits of owl species. This study presents the first comprehensive comparison of pellet dimensions—specifically length, breadth, and mass—across all 11 resident owl species in Israel. A total of 816 pellets were collected from diverse habitats, including Mediterranean woodlands, agricultural landscapes, and arid deserts. Pellet measurements were analyzed in relation to the average body length of each species, revealing significant interspecific variation in all three dimensions. Statistical analyses confirmed strong positive correlations between body size and pellet length (r = 0.95), breadth (r = 0.91), and mass (r = 0.96), highlighting the influence of morphological constraints on pellet structure. Larger owls, such as Bubo bubo and B. ascalaphus, produced the largest pellets, whereas smaller species, such as Otus brucei and O. scops, generated notably smaller and lighter pellets, consistent with their known dietary preferences. Habitat differences and ecological specialization likely contribute to further variability in pellet morphology, even among closely related taxa. By focusing on pellet morphometrics rather than prey composition, this study offers a standardized and replicable method for interspecific comparisons. The findings support the use of pellet size as a proxy for predator body size and ecological strategies and provide a valuable baseline for future research on owl diets, habitat use, and species identification in the Middle East and elsewhere. This study enhances the utility of pellet analysis in both ecological monitoring and conservation biology.

1. Introduction

Owls, as obligate carnivores and nocturnal predators, occupy a crucial ecological niche within terrestrial ecosystems across the globe. Their widespread distribution, species richness, and broad dietary range make them important indicators of environmental health and biodiversity. In Israel, a biogeographically diverse region at the intersection of three continents—Africa, Asia, and Europe—owls exhibit a particularly rich assemblage [1]. This diversity, driven by the country’s varied habitats and climatic gradients, offers a unique opportunity to comparatively study ecological traits among coexisting species.

Among the many ways to assess the ecology of owls, pellet analysis remains one of the most informative and non-invasive methods for understanding their feeding behaviors, habitat use, and interspecific interactions [2]. Owl pellets, composed of undigested remains such as bones, fur, and exoskeletons, provide valuable data on prey composition and allow researchers to draw inferences about an owl’s hunting strategies, niche occupation, and health status [3] and even assess environmental contamination [4,5] and other anthropogenic impacts [6]. While prey identification is the most commonly pursued objective of pellet studies [7,8], the morphometric parameters of the pellets themselves—such as length, breadth, and mass—can offer additional insights into species-level anatomical and physiological traits, especially when examined across a wide taxonomic range. Pellet analysis also enables genetic analyses of both predators and their prey [2,9].

In the Middle East—a region encompassing a wide range of ecosystems, ranging from deserts and woodlands to wetlands and agricultural landscapes—many owl species regularly produce pellets that reflect their highly variable diets, typically consisting of small mammals, birds, reptiles, and insects [10,11]. Studies from Israel, Jordan, Iran, and Turkey have used pellet contents to document regional diet patterns, showing that many owls in this area primarily consume rodents, making them important contributors to natural pest control [12]. Because the composition of owl pellets is influenced by habitat type and local prey communities, pellet analysis has also proven useful for biodiversity monitoring and as a proxy for small-mammal surveys [13,14].

Interspecific differences in pellet size and composition have been linked to variations in owl body size [15], hunting strategies [16], and ecological niches [17,18,19]. Seasonal and geographic fluctuations in prey availability can further shape diet composition, indicating that owls are responsive to changes in their environment [20]. Although increasing attention is paid to owl diet studies across the Middle East, large parts of the region remain poorly sampled, and further research is needed to better understand the ecology and trophic roles of local owl species [21]. While biases may arise in prey identification based on pellet contents [22], this method’s simplicity, non-invasiveness, and relatively low cost have made it a cornerstone of avian ecological research [23]. Nonetheless, most published studies to date focus on single species or specific sites [24,25], or on diet and niche competition [14,21], limiting broader comparative insights.

Despite the rich avian diversity in Israel and the long-standing interest in owl biology, few studies have systematically documented pellet morphometry across the multiple species distributed along the country’s complex environmental gradient. Israel’s landscapes range from Mediterranean woodlands and agricultural plains in the north and center to hyper-arid deserts in the south. These contrasting ecosystems host distinct owl communities, often exhibiting site-specific adaptations. For instance, the Tawny Owl (Strix aluco) is restricted to the wetter, forested north, while the Pharaoh’s Eagle Owl (Bubo ascalaphus) and the Desert Little Owl (Athene lilith) are found in the arid south. This ecological segregation provides a natural laboratory for investigating how environmental pressures and body size affect pellet dimensions across taxa.

The present study addresses this gap by compiling and analyzing a large, geographically representative dataset of owl pellets collected from across Israel. Our sampling effort encompassed 816 pellets from 11 breeding owl species, spanning a latitudinal range of approximately 640 km—from the Upper Galilee to the Arava Rift Valley. We aimed to characterize the variation in pellet size (length, breadth, and mass) among these species and assess the extent to which pellet dimensions correlate with body size. In doing so, we sought to establish a morphometric baseline for owl pellets in Israel and contribute to the broader understanding of predator–prey interactions and morphological scaling in regard to raptors.

In this study, we present and compare the dimensions of randomly collected pellets from all 11 resident owl species in Israel. These include two species pairs with similar ecological roles but differing habitat preferences: the Eurasian Eagle Owl (Bubo bubo) and its desert counterpart, the Pharaoh’s Eagle Owl (Bubo ascalaphus), and the Tawny Owl (Strix aluco) and Desert Tawny Owl (Strix hadorami). The other species are the Barn Owl (Tyto alba), Long-Eared Owl (Asio otus), Short-Eared Owl (Asio flammeus), Little Owl (Athene noctua), Desert Little Owl (Athene lilith), Eurasian Scops Owl (Otus scops), and Striated Scops Owl (Otus brucei). By examining variation in pellet size across this full suite of species, we aim to contribute to a broader understanding of owl ecology and interspecific differentiation in feeding ecology across arid and Mediterranean environments.

2. Methods

Recent advancements in Strigiformes taxonomy have led to updates in species identification and comparative research [26]. This study follows the taxonomic framework of the International Ornithologists’ Union (IOU), as presented in the IOC World Bird List, which incorporates recent molecular and morphological findings [26,27]. Molecular evidence has elevated several previously grouped taxa to full species, including Pharaoh’s Eagle Owl (Bubo ascalaphus) and the Desert Tawny Owl (Strix hadorami). We also treat the Desert Little Owl (Athene lilith) as a distinct species based on prior studies [24,26,28], although its classification remains debated [29]. These taxonomic updates reflect a more refined understanding of species boundaries and guide our classifications, ensuring consistency and scientific accuracy.

The geographic distribution of owl species in Israel is largely determined by habitat preferences, and, as such, pellet samples were not collected synchronously across space or time. Instead, collection sites spanned the full north–south gradient of the country, covering approximately 640 km, ranging from the Upper Galilee in the north to the Arava Rift Valley and Negev Desert in the south. Pellets of the Tawny Owl were collected in the Upper Galilee region, characterized by Mediterranean woodland habitats. Barn Owl pellets were collected from nest box colonies erected in agro-landscapes in the Judea Region. Pellets of the Eurasian Eagle Owl, Long-Eared Owl, Short-Eared Owl, and Eurasian Scops Owl were collected in the Judea region of Central Israel, which includes a mosaic of Mediterranean scrubland and planted forests [10]. Desert Tawny Owl pellets were collected in the dry canyons near Ein Gedi in the Judean Desert. Pharaoh’s Eagle Owl and Pale Scops Owl pellets were collected in the Arava Rift Valley, a hyper-arid region south of the Dead Sea. Pellets of the Little Owl were collected from nests in the Western Negev and Central Israel; and pellets of the Desert Little Owl were collected in the Negev Desert highlands, near Mitzpe Ramon. Most pellets were collected at nest sites or species-specific communal roosts used during the non-breeding season. In the field, pellets were individually placed in sealed Ziplock bags, labeled, and transported to our laboratory.

Standardized laboratory procedures were employed to measure dry pellet dimensions with high precision. Each of the 816 pellets of the 11 owl species was oven-dried, measured for length and breadth using a digital caliper (±0.1 mm), and weighed with an electronic balance (±0.1 g) (

Table 1. Dimensions of the pellets of the owls of Israel and midpoints of their body-length variability intervals.

Species	No. of Pellets	Length (mm)		Breadth (mm)		Mass (g)		Body Length (cm)
		Mean (CL)	Range	Mean (CL)	Range	Mean (CL)	Range
Asio flammeus	68	57.8 (53.9–61.7)	21.8–87.8	19.9 (19.3–20.5)	13.3–25.2	3.3 (3.0–3.6)	1.0–4.8	38.0
Asio otus	70	48.7 (45.9–51.5)	20.6–75.0	19.2 (18.6–19.9)	11.3–25.6	3.0 (2.8–3.3)	1.2–4.8	35.5
Athene lilith	151	39.2 (38.1–40.2)	20.2–58.5	14.4 (14.2–14.6)	10.5–21.1	2.2 (2.1–2.4)	0.3–7.3	23.0
Athene noctua	102	39.3 (37.9–40.6)	20.0–57.6	14.2 (13.9–14.5)	10.0–18.5	2.0 (1.9–2.1)	0.4–4.7	22.0
Bubo ascalaphus	52	62.4 (57.9–67.0)	40.0–100.0	29.1 (28.0–30.2)	19.5–40.3	9.0 (8.0–10.1)	3.4–20.0	48.0
Bubo bubo	100	79.4 (74.1–84.7)	48.5–160.0	30.9 (30.2–31.5)	23.0–50.5	13.1 (12.2–14.1)	6.7–28.0	66.5
Otus brucei	31	23.8 (22.6–24.9)	19.0–31.4	11.3 (10.9–11.6)	9.6–12.8	0.8 (0.7–1.0)	0.3–1.6	20.0
Otus scops	36	25.4 (24.1–26.8)	19.2–36.2	12.1 (11.4–12.7)	9.1–16.2	0.9 (0.8–1.0)	0.4–1.9	20.0
Strix aluco	52	53.3 (50.1–56.5)	28.8–73.9	27.0 (25.8–28.3)	17.0–35.0	6.1 (5.5–6.7)	2.5–11.4	40.0
Strix hadoramii	26	35.7 (32.7–38.6)	23.0–46.1	15.4 (14.5–16.3)	11.9–19.2	2.5 (2.1–2.8)	1.1–3.9	29.5
Tyto alba	136	46.8 (45.1–48.5)	29.7–74.0	26.3 (25.9–26.8)	19.1–33.9	4.7 (4.5–5.0)	1.9–8.5	36.0

). To minimize observer bias, a single researcher performed all measurements. Body length data for each species were drawn from authoritative literature sources [27,30,31] and used as a proxy for overall body size in subsequent analyses. These data allowed us to examine scaling relationships between body size and pellet dimensions, thereby contributing to the emerging field of morphological allometry regarding birds of prey. The comparison of pellet dimensions between species was performed using MANOVA and one-way ANOVA. Possible relationships between pellet dimensions and owl body length were analyzed using Pearson’s correlation. Throughout the text, mean values are presented alongside 95% confidence limits (CLs).

3. Results

Statistical analyses revealed significant interspecific variation in all the pellet parameters measured (Table 1). The multivariate analysis of variance (MANOVA) confirmed that pellet length, breadth, and mass differed substantially among the 11 owl species (Wilks Lambda = 0.047, F_{30, 2357} = 143.87, p < 0.001). One-way ANOVAs conducted separately for each variable supported these findings, showing highly significant differences across species for length (F_{10, 805} = 100.14), breadth (F_{10, 805} = 538.26), and mass (F_{10, 805} = 234.29), all with p-values less than 0.001 (Figure 1).

More importantly, we found robust positive correlations between the average pellet dimensions and the body length of the respective owl species. Pearson correlation coefficients were exceptionally high for all three dimensions, namely, length (r = 0.95), breadth (r = 0.91), and mass (r = 0.96), each statistically significant at p < 0.001 (Figure 2).

4. Discussion

This study presents the first comprehensive analysis of pellet morphometrics across all 11 resident owl species in Israel, spanning a broad range of habitats and ecological contexts. Our results demonstrate clear interspecific differences in pellet size—specifically length, breadth, and mass—which correlate strongly and positively with the body sizes of the respective species. These findings support previous suggestions that pellet dimensions reflect predator morphology and physiology [15,16] and further highlight the utility of pellet analysis as a multifaceted tool in avian ecological research.

A comparison of the pellet measurements among the different owl species reveals considerable variation and overlap across species and geographic regions (

Table 2. Comparison of average owl pellet dimensions (length × breadth, in mm) recorded in this study with values reported in previous studies. Differences in pellet size may reflect regional variation in diet, prey size, and owl body condition. For some species (Athene lilith, Otus brucei, and Strix hadorami), no published measurements were available, highlighting gaps in current knowledge.

	Our Study: Average Pellet Size	Others: Average Pellet Size	Source
Asio flammeus	58 × 20	49 × 22	[32,33,34,35,36]
Asio otus	49 × 19	75 × 30	[12,33,37,38,39]
Athene lilith	39 × 14		No data
Athene noctua	39 × 14	25 × 15	[29,35]
Bubo ascalaphus	62 × 29	47 × 25	[40]
Bubo bubo	79 × 30	75 × 32	[10,33,41]
Otus brucei	24 × 11		No data
Otus scops	25 × 12		[42]
Strix aluco	53 × 27	60 × 30	[33,41,43,44]
Strix hadoramii	36 × 15		No data
Tyto alba	45 × 26	50 × 30	[33,45,46,47]

). These results highlight the importance of considering ecological and regional factors when analyzing dietary patterns derived from pellet studies.

One of the main gaps in methodology in many of the pellet-based dietary studies is the absence of standardized measurements of the pellets obtained before analyzing their contents. As a result, numerous studies rely primarily on qualitative observations, with size differences often inferred based on the assumed relationship between an owl’s body size and its prey selection [14,21]. This hinders the comparability of data between studies and obscures significant ecological patterns in pellet morphology.

The strong correlations between body size and pellet parameters observed in this study suggest that morphological and anatomical constraints are major drivers of pellet characteristics, in addition to dietary composition. Larger owls, such as the Eurasian Eagle Owl and Pharaoh’s Eagle Owl, produced substantially larger and heavier pellets, a finding is consistent with their consumption of larger vertebrate prey. Conversely, smaller owls such as the Eurasian Scops Owl and Pale Scops Owl, which primarily consume insects and small invertebrates, produced notably smaller and lighter pellets, in line with their trophic specialization [17,18].

Importantly, our study reveals that the variation in pellet morphology is not solely a function of body size but also reflects ecological diversity. The broad environmental gradient sampled—ranging from the humid, forested north to the hyper-arid deserts of the south—captures a range of prey communities, vegetation types, and microhabitats. These factors can indirectly influence pellet size by shaping the prey base available to each species as well as hunting strategies and energetic requirements. For example, owls in desert regions may target different prey taxa or show dietary specialization based on seasonal scarcity, which could influence both the composition and compactness of their pellets [19,20].

Interestingly, we observed significant differences in pellet size even between closely related or morphologically similar species, such as the Tawny Owl and the Desert Tawny Owl. These distinctions likely reflect fine-scale adaptations to habitat structure and prey availability, reinforcing the notion that ecological divergence within taxonomic groups can manifest in measurable morphological traits.

Nonetheless, several limitations should be acknowledged. First, because pellet collection was not temporally or spatially synchronized, local environmental conditions and temporal fluctuations in prey populations may have introduced some variation in pellet size and structure. Second, natural differences in pellet compaction, moisture content at the time of collection, or variation in prey digestibility could contribute to unexplained variability. Although all our measurements were performed under standardized laboratory conditions by a single observer to reduce measurement error, some residual variation was inevitable. Third, we relied on literature-based body length data, which may not capture the full range of intra-population variation within each species.

To address these limitations and build upon our findings, future research should combine morphometric analyses with prey content identification and dietary breadth metrics across seasons and ecological zones. Integrating pellet data with environmental variables—such as temperature, prey availability, or habitat fragmentation—could also help clarify how external pressures shape predators’ foraging strategies and digestive output. Additionally, longitudinal studies that track pellet production over multiple breeding seasons may reveal temporal trends in response to climate variability or anthropogenic change.

This study also has potential applications beyond basic ecology. As owl pellets are often collected for educational purposes or as part of conservation monitoring programs, the morphometric framework provided here can inform rapid field assessments or citizen science initiatives. Moreover, because owl pellets are preserved in cave sites and archaeological contexts [48], our findings may contribute to paleoecological reconstructions, offering insights into past faunal assemblages and predator–prey dynamics in the Levant and other arid regions.

In conclusion, our study demonstrates robust species-level differences in owl pellet morphology in Israel and highlights the strong relationship between predator body size and pellet dimensions. These patterns reflect both anatomical constraints and ecological adaptations, reinforcing the value of pellet analysis as a tool for studying avian trophic ecology, morphological scaling, and habitat specialization. By establishing a comparative baseline across Israel’s diverse owl fauna, we provide a reference point for future investigations into raptor foraging behavior, species identification, and environmental monitoring across the Middle East and similar ecotones worldwide.