Male and female black-and-chestnut eagles feed their chicks differently.

This study provides the first analysis of sex-related dietary patterns in the globally Endangered black-and-chestnut eagle (Spizaetus isidori), one of the largest aerial predators of Andean mountain forests. The authors examined whether reversed sexual size dimorphism (females 3.5–4 kg; males 1.6–2.7 kg) translates into differences in prey size, prey delivery rates, and prey diversity between the sexes during the nestling-rearing period.

Methods

Direct observations were conducted at nine nests across the Ecuadorian Andes between 2017 and 2024, with intensive monitoring at the El Salado nest (1,578 m a.s.l.) from May to August 2024. Observations were made from ~50 m distance using binoculars, telescopes, and cameras. The dataset comprised 326 prey items: 16 recorded in 2024 plus 310 items from published sources (2017–2023). Prey were classified by species, body mass (from literature), and habit (terrestrial, arboreal, scansorial). Statistical analyses included Levins' standardized food-niche breadth, Pianka's niche overlap index with null-model simulations (EcoSimR), chi-squared tests with Monte Carlo integration, and Mann–Whitney U tests.

Key Findings

Prey delivery rates: Males delivered more than twice as many prey items to nestlings as females (227 vs. 99), a significant difference (χ² = 45.74, p = 0.02). This aligns with patterns documented in harpy eagles and crowned eagles.

Prey size: Females captured significantly heavier prey than males (mean 1.19 kg vs. 0.95 kg; p = 0.02). Female prey included larger mammals such as Linnaeus's two-toed sloth (6.30 kg) and southern tamandua (6.05 kg), while male prey ranged from the Amazon dwarf squirrel (0.10 kg) to the southern tamandua (6.05 kg).

Prey diversity: No significant difference was found between sexes in Levins' standardized food-niche breadth (males p = 0.43; females p = 0.16), nor in species composition when considering only main prey items (≥5% biomass; χ² = 7.72, p = 0.16). However, when prey with lower biomass contributions were included, a significant difference emerged (χ² = 45.74, p = 0.02).

Niche overlap: Pianka's index was 0.67, indicating relatively high overlap in captured prey between sexes, significantly higher than expected by chance (p = 0.05), suggesting potential for competition.

Taxonomic differences: Males preyed primarily on birds (Galliformes, Columbiformes, Psittaciformes) and rodents, while females preyed mostly on mammals (Primates, Carnivora, Pilosa) and some passerines. Both sexes preferred arboreal prey, but males hunted more scansorial and terrestrial species.

Conclusions

The results do not support the intersexual competition hypothesis as an explanation for reversed sexual dimorphism in this species. Instead, the data support the parental role differentiation hypothesis: smaller males are more efficient foragers, delivering more prey and hunting more agile, diverse prey, while larger females capture heavier, potentially more dangerous mammalian prey. The authors note that prey transfers between mates may bias estimates of intersexual prey selection, and recommend further research using nest cameras to quantify temporal changes in prey delivery and size across nesting stages.

Conservation Implications

The black-and-chestnut eagle is listed as globally Endangered, with fewer than 250 adults estimated in Ecuador (nationally Critically Endangered). The species regularly preys on domestic poultry, contributing to human–wildlife conflict and retaliatory shooting. Understanding sex-specific dietary requirements is fundamental for effective management and conservation planning. The authors emphasize that human persecution remains one of the greatest threats to the species' survival in the northern Andes.

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Photo: Jaime Culebras