Introduction

Predation patterns can be affected by a wide range of ecological constraints that vary across the geographic range of predators1. In raptors, size-related differences between sexes affect the prey these predators consume2,3. The phenomenon of reversed sexual dimorphism4—where females are larger than males—in raptors occurs either because large females are better brooders, and smaller males are better foragers and territory defenders, or as a mechanism to reduce intersexual competition for food since both sexes forage in the same territory to provide prey for their nestlings5,6. This phenomenon is pronounced in the black-and-chestnut eagle (Spizaetus isidori; Fig. 1), one of the largest aerial predators of mountain forests in the Andes and the Sierra Nevada de Santa Marta in South America7. Females weigh between 3.5 and 4 kg, while males weigh between 1.6 and 2.7 kg8,9.

Fig. 1
figure 1

Black-and-chestnut eagle (Spizaetus isidori) arriving at an active nest in Ecuador (photo: Daniel Mideros).

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The black-and-chestnut eagle nests at an altitudinal range from 1578 to 2727 m above sea level10,11 and their nests and surrounding landscapes in the northern Andes consist of varying proportions (12.8–98.2%) of forest cover11. Egg-laying and incubation occur throughout the year in the tropical Andes11 and between August and December in the subtropical Andes10. The total eagle population is estimated to be between 1400 and 4200 adults and is in decline12. The species is therefore listed as globally Endangered12. However, due to human threats, mainly habitat loss and poaching, but also electrocution on power lines, illegal trafficking, and collision with vehicles11,13 its population size in Ecuador has been estimated at fewer than 250 adults14. The species is, therefore, nationally classified as Critically Endangered14.

The species feeds on a variety of prey, including large birds, arboreal mammals, and reptiles10,11,15. An analysis of 853 prey items delivered to nestlings in 16 nests located in the northern Andes indicated that the black-and-chestnut eagle regularly preyed on poultry, with biomass contributions (14.57% ± 10.55) representing 0.6–37% of the total prey consumed11. The eagle is an adaptable generalist capable of switching from mammalian carnivores to guans in human-transformed Andean landscapes in Ecuador and Colombia11. As one study found, eagles nesting in sites with low forest cover had a less diverse diet than those in areas with more intact forests11. The Restrepo-Cardona et al.11 study focused on analyzing how forest cover affects the feeding habits of the black-and-chestnut eagle during nestling-rearing periods and how high predation rates on poultry have contributed to the human-black-and-chestnut eagle conflict16,17 as well as the subsequent shooting and capture of the species in response to poultry predation13,16.

While a comprehensive understanding of dietary requirements is fundamental for the effective management and conservation planning of this globally endangered species, specific knowledge of the sex-related dietary differences in black-and-chestnut eagles remains unknown. In this study, we expand on Restrepo-Cardona et al.11 and examine the effects of the species’ sexual dimorphism on their feeding ecology during nestling-rearing periods. We aimed to evaluate the differences in prey size between female and male eagles, as well as the numerical rates of prey delivery to nestlings, and the differences in prey diversity between sexes, in nine nests in Ecuador. We predicted that male black-and-chestnut eagles would provide a greater number of prey items to nestlings than females, and that prey weight would be higher for female eagles than for males, but males would hunt a more diverse range of prey than females2,5,18,19,20.

Methods

Study area

This study was conducted between 2017 and 2024, in nine black-and-chestnut eagle nests in the Andean region of Ecuador (Fig. 2). Four nests were located in the Napo province, (1) Parada Larca at 2,187 m a.s.l. (0°26’S, 77°53’W) in the middle basin of the Río Parada Larca, and (2) El Salado at 1,578 m a.s.l. (0°10’S, 77°41’W) in the middle basin of the Río Salado. Both sites are in the buffer zone of the Cayambe Coca National Park. (3) Quijos Huaico at 2,259 m a.s.l. (0°26’S, 77°58’W) in the middle basin of the Río Quijos Huaico, and (4) Cuyuja at 2,632 m a.s.l. (0°25’S, 78°1’W) in the middle basin of the Río Papallacta. Both sites are located within the buffer zone of Antisana National Park. Three nests were located in the Tungurahua Province, (5) Río Blanco at 2,229 m a.s.l. (1°22’S, 78°20’W) and (6) El Triunfo at 2,452 m a.s.l. (1°19’S, 78°24’W). Both sites are in the middle basin of the Río Pastaza, and (7) Zuñag at 1,800 m a.s.l. (1°22’S, 78°9’W) in the middle basin of the Río Zuñag. All three sites are within the Llanganates Sangay wildlife corridor, in the buffer zone of the Llanganates and Sangay National Parks. One nest was in the Pichincha Province, (8) Atahualpa, at 2,727 m a.s.l. (0°9’N, 78°19’W) in the Área de Conservación y Uso Sustentable Mojanda Cambugán, in the middle basin of the Río Mojanda, and another nest was in the Morona Santiago Province, (9) Chimandaz at 1,785 m a.s.l. (3°9’S, 78°20’W) in the middle basin of the Río Zamora11. The eagle nests and their surrounding landscapes consist of varying proportions (33.6–98.2%) of forest cover, heterogeneous agricultural areas, cattle pastures, herbaceous or shrubby vegetation, transitional or permanent crops, man-made bodies of water, urban areas, and industrial zones11.

Fig. 2
figure 2

Map of the locations of nine black-and-chestnut eagle (Spizaetus isidori) nests in the Ecuadorian Andes. (1) Parada Larca, (2) El Salado, (3) Quijos Huaico, (4) Cuyuja, (5) Río Blanco, (6) El Triunfo, (7) Zuñag, (8) Atahualpa, and (9) Chimandaz. The map was created using ArcGIS 10.8 software (URL: https://www.arcgis.com/index.html).

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Data collection and acquisition

To evaluate the sex-related dietary patterns in the black-and-chestnut eagle during the nestling-rearing period, we conducted systematic observations at El Salado nest between May and August 2024. Direct observations at the nest were performed by trained technicians using binoculars (10 × 42 and 10 × 50), telescopes (20–60 × 60 and 20–60 × 65), and photographic cameras, from high observation points at a horizontal distance of approximately 50 m from the nest. Observations were made between 0600 and 1800 h at the nest. We complemented our dataset with other published data on prey items recorded in El Salado and another eight eagle nests in the Ecuadorian Andes between 2017 and 2023, in which prey items were also recorded through direct observations during nestling-rearing periods11. These additional data (n = 310 prey items) were obtained by conducting a bibliographic search performed in Google Scholar and Scopus using the keywords: Spizaetus isidori, black-and-chestnut eagle, and águila andina, combined with diet, feeding habits, dieta, hábitos de alimentación, and Ecuador.

Prey items were identified to the finest possible taxonomic level from photographs using bird and mammal guides21,22,23,24. Mean body masses of prey species were obtained from the literature8,21,22,25. We classified the black-and-chestnut eagle prey, dividing the species into terrestrial, arboreal, and scansorial21,22,26,27.

Data analysis

To evaluate the prey diversity of male and female black-and-chestnut eagles, the Levins’ standardized food-niche breadth28 index was calculated: Bsta = B−1/(n − 1), where B is the Levins’ index: B = 1/Σpi2, where pi is the percentage of each prey category, and n is the total number of prey categories29. The values of this index range from 0 (minimum niche breadth, which implies maximum selectivity) to 1 (maximum niche breadth, minimum selectivity)30. To examine the overlap in captured prey between male and female eagles, we calculated the Pianka ́s index31: O = Σpiqi/√Σpi2Σqi2, where pi is the frequency of a prey item in the diet of one sex, and qi is the frequency of the same prey item in the diet of the other sex. This index’s values range from 0 (no overlap) to 1 (complete overlap). For this last analysis, we employed a null-model approach, which enabled us to calculate the overlap and the probability of it being higher or lower than expected by chance. We employed the EcoSimR32, performing 1000 simulations using randomization algorithms 3 and 4 (RA3 and RA4, respectively). RA3 randomizes all row values, whereas RA4 randomizes only the non-zero row values. Both algorithms preserve the observed “niche breadth” of each species (i.e., relative level of specialization), but they randomly modify the specific resource categories utilized.

We tested for differences in observed food-niche breadth between sexes using a null model approach. To do this, we generated a frequency distribution for the standardized Levin’s index for each sex and their differences by subsampling, with replacement, 20 prey items 1,000 times. We then compared this distribution with the observed value to estimate how much the differences deviate from what would be expected by chance alone. We selected 20 prey items because this number was the most optimal for accounting for the major biomass contribution in the same dataset11. Additionally, we performed a Chi-squared (χ²) test to examine differences in abundance and habits of species hunted by male and female eagles, while a Mann-Whitney U test was used to examine differences in the mean prey size. Because the Chi-squared test requires large sample sizes and relies on asymptotic approximations, we opted to use its integration with the Monte Carlo method, which addresses those limitations by generating empirical distributions that relax the traditional assumptions of the test. All tests were performed using R software version 2.133. We considered the results to be statistically significant when p ≤ 0.05.

Results

Regarding the Levins’ standardized food-niche breadth, it was not possible to differentiate the values for males (p-value = 0.43) and females (p-value = 0.16) from those of a random prey composition (Fig. 3). There was also no difference in the species composition of the captured prey between sexes (χ2 = 7.72, df = NA, p-value = 0.16) when considering only species that contribute 5% or more of the total biomass (i.e., red-tailed squirrel Sciurus granatensis, black agouti Dasyprocta fuliginosa, South American coati Nasua nasua, sickle-winged guan Chamaepetes goudotii, Andean guan Penelope montagnii, domestic chicken Gallus gallus11). However, when prey with lower biomass contributions were included, a significant difference emerged (χ2 = 45.74, df = NA, p-value = 0.02), indicating that these prey items were captured differently.

The value of Pianka’s niche overlap between male and female black-and-chestnut eagles was 0.67, indicating a relatively high overlap in captured prey between sexes. Furthermore, compared to simulated values, there was a significant difference from what was expected by chance (p-value = 0.05), given that 75% of the simulated values were below 0.65.

Fig. 3
figure 3

Density distribution of Bsta values for male and female black-and-chestnut eagles (Spizaetus isidori) (A) and the absolute difference between them (B) generated from null models. Dots and dashed lines indicate the observed values (Bsta.obs and Bsta.diff), with the p-value representing the area under the curve to the right.

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Male black-and-chestnut eagles provided more prey items to nestlings than females (227 vs. 99, respectively) (χ2 = 45.74, df = NA, p-value = 0.02). In addition, males captured prey that were, on average, smaller (0.95 kg) than prey captured by females (1.19 kg), with statistically significant differences (p-value = 0.02) (Fig. 4). Male eagles prey on at least 22 species, with weights ranging from 0.10 kg of the Amazon dwarf squirrel (Microsciurus flaviventer) to 6.05 kg of the southern tamandua (Tamandua tetradactyla). Females prey on at least 18 species, with weights ranging from 0.11 kg of the eared dove (Zenaida auriculata) to 6.30 kg of the Linné’s two-toed sloth (Choloepus didactylus) (Table 1).

Fig. 4
figure 4

Frequency distribution of prey weights delivered by male and female black-and-chestnut eagles (Spizaetus isidori). Outlier values were capped for better visualization.

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Table 1 Prey items delivered to nestlings by males and females of nine black-and-chestnut eagle (Spizaetus isidori) pairs in the Ecuadorian Andes. In this study, we included 16 prey items recorded in El Salado nest in 2024, as well as 310 prey items recorded in El Salado and eight additional eagle nests between 2017 and 2023, as reported by Restrepo-Cardona et al.11. The analysis include weight (W), habit (terrestrial, arboreal, scansorial), and number of prey (N).
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Moreover, although there is no statistically significant difference in the habits of hunted prey (χ2 = 4.55, df = NA, p-value = 0.09), trends can be observed. In species in which black-and-chestnut eagles appeared to have differential capture, both males and females preferred arboreal species, with males also preying on more scansorial and terrestrial prey. However, males mainly captured birds and rodents, while females tended to capture more mammals and some passerine birds (Fig. 5).

Fig. 5
figure 5

Matrix of differences in prey capture between male and female black-and-chestnut eagles (Spizaetus isidori) with contributions of less than 5% of total biomass. The color scale indicates the absolute difference between the abundance of prey hunted by males and the abundance of prey hunted by females. White cells represent pseudo-zeros (NA values) necessary to complete all the possible combinations in the matrix.

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Discussion

This study provides the first analysis of sex-related dietary patterns in the black-and-chestnut eagle. Our findings indicate that male eagles provided nestlings with more than double the amount of prey compared to females (Table 1). Similarly, male harpy eagles (Harpia harpyja) and crowned eagles (Stephanoaetus coronatus) provided most of the food for their nestlings2,19. Female black-and-chestnut eagles preyed on larger prey species than males (Fig. 4), likely due to the predation of mammal species from the orders Carnivora, Primates, and Pilosa. Female Eurasian sparrowhawks (Accipiter nisus) and harpy eagles hunted heavier prey than males18,20. Although mammalian carnivores can use their claws against predators, primates and sloths rely on anchoring themselves to branches to avoid predation1,2 and porcupine quills can cause injury or death to at least nine raptor species34; due to their large size, female black-and-chestnut eagles may be able to capture and handle them more effectively than males. Furthermore, by hunting small prey in greater numbers, male eagles can obtain more energy per unit of time invested in foraging and reduce the carrying costs from capture sites to nests2,35,36.

There is no difference between male and female black-and-chestnut eagles in either prey diversity (Fig. 3) nor the composition of the prey species when considering the main prey items. However, the overlap in captured prey between the sexes was relatively high, suggesting potential for competition. The intersexual competition hypothesis proposes that central place foragers such as raptors exploit different prey niches to minimize competition for food resources during the nestling period6. Our results, therefore, do not support intersexual competition reduction as an explanation for reversed sexual dimorphism in the species. Instead, our data support the hypothesis of parental role differentiation, as small males appear to be more efficient foragers than females6. Male black-and-chestnut eagles prey on more scansorial and terrestrial animals than females, even though both sexes prefer arboreal species (Table 1; Fig. 5). This suggests that males require more agility than females, which is likely related to their smaller body size. More agile black-and-chestnut eagle males are likely better able to prey on a suite of species that vary in their habit (i.e., terrestrial, arboreal, scansorial). Smaller-bodied male raptors overall have more agility compared with females5. For example, the smaller size of male harpy eagles enables them to feed on a more diverse range of prey and hunt more terrestrial animals. Additionally, the smaller size gives them the agility to maneuver through the layers of lianas and branches from the canopy to the ground2,18.

Male eagles captured more domestic chickens (terrestrial prey) than females, although the difference was not significant (Table 1). The black-and-chestnut eagle has also been recorded consuming other poultry species in Colombia, such as domestic turkeys (Meleagris gallopavo)11 and ducks (Anas platyrhynchos, Cairina moschata) (Authors’ unpublished data). Consumption of terrestrial prey by male eagles can be detrimental to the population´s sex ratio, as predation of domestic animals may be more common among males, resulting in higher rates of retaliatory shooting by local people and thus higher male mortality2. This is particularly concerning in Ecuador, where human persecution as retaliation or as a preventive measure against poultry predation poses one of the greatest threats to the conservation of the species. Between 2000 and 2022 alone, 24 eagles were poached in Ecuador13. Enforcing anti-persecution laws is key to stopping the human persecution of raptors in the northern Andes13,37,38,39,40. In 2023, a person was sentenced to 12 months in jail and ordered to pay a USD 1700 fine in Ecuador for being responsible for shooting and killing an adult black-and-chestnut eagle in the province of Napo, thus establishing a precedent for the application of environmental law in the region.

We observed that male black-and-chestnut eagles often transferred captured prey to females within the nests, likely because female raptors are well-known as the primary food processors for nestlings41. While we did not observe this behavior outside the nests, it remains possible that such prey transfers between males and females occurred beyond our field of observation. Therefore, our results should be interpreted with caution. If the male’s allocation of a prey item directly to the nestlings or indirectly via the female depends on prey type or size, any estimate of an intersexual difference in prey selection would be biased35.

Furthermore, we did not analyze how diet varies with nestling age, as conducting the observations on consecutive days while eagles fed their nestlings in the nests was not feasible. Male raptors hunt most often during the initial breeding phase, when the females are incubating, brooding, and depending on their mates for food41. As the nestlings age, females proportionally increase their prey provisioning rates at a higher rate relative to males19. Other techniques for studying raptor diets42,43,44,45 such as nest camera analysis18,19,36 could help evaluate temporal changes in delivery frequency and prey size during the nestling stage. Continuous monitoring with nest cameras reduces biases in diet composition by recording all prey deliveries18,19,46. We acknowledge that our study has some limitations. However, our results reveal novel insights into the feeding habits of this globally endangered aerial predator and provide evidence to support the effects of sex on the raptors’ dietary contributions to nestlings.

Conclusion

Knowledge gaps remain regarding the differences in how male and female raptors allocate food resources. Our findings showed that, as hypothesized, male black-and-chestnut eagles provided more prey to nestlings and fed on smaller prey than females. The overlap in captured prey between sexes was relatively high, suggesting non-sex-specific partitioning in resource use. Furthermore, the standardized Levins food-niche breadth values indicate no difference between male and female eagles in selecting the main prey species. However, the sexes differed in the capture of prey items with lower biomass contributions. Males’ prey were primarily bird species from the orders Galliformes, Columbiformes, and Psittaciformes, while females’ prey were mostly mammal species from the orders Primates and Carnivora, and some passerine birds. These data suggest that sex-related differences in the eagle’s dietary contribution to nestlings likely result from parental role differentiation, rather than intersexual competition. However, further research should be conducted to achieve a comprehensive understanding of the sources of the asymmetric parental role in the black-and-chestnut eagle’s feeding habits.