Haematological and biochemical reference values for Bonelli's eagles in the wild and in captivity: implications for conservation and rehabilitation programs

Abstract

Background: Haematological and clinical biochemistry analyses are essential tools for evaluating the health status of avian species, including the endangered Bonelli's eagle. Despite their importance, existing reference intervals (RIs) for such parameters in raptor species are frequently constrained by limited sample sizes, thus complicating clinical interpretations. This research followed the American Society for Veterinary Clinical Pathology (ASVCP) guidelines to establish haematological and clinical biochemistry reference intervals for the Bonelli's eagle, utilizing an indirect approach. Conservation initiatives of the Bonelli's eagle have provided a unique opportunity to gather a high number of samples under various conditions, facilitating a comprehensive comparison between wild and captive populations. Over the course of six years, from 2016 to 2021, our research analyzed 516 blood samples collected within the framework of a European Life Project. After rigorous data cleaning and stringent selection criteria application, a representative sample of 184 birds was determined. Various physiological parameters and blood lead levels were quantified in healthy individuals. Additionally, we used Principal Component Analysis to discern distinctions between populations and to investigate potential interrelationships among the diverse parameters.

Results: We established Reference Intervals for three distinct reference groups of birds: wild nestlings ( $n=72$ ), captive nestlings ( $n=45$ ), and adults ( $n=49$ ). Our findings indicate that age significantly affects many blood parameters. Meanwhile, gender impacts only a few parameters in adult birds. Notably, wild nestlings exhibited higher levels of AST, K, TP, and Pb in their blood compared to those bred in captivity.

Conclusions: This paper provides the first reliable RI for physiological, haematological, and clinical biochemistry parameters in both nestling and adult Bonelli's eagles. The data will augment the knowledge of the physiology of this endangered raptor, contributing significantly to the understanding and monitoring of both free-living birds and captivity programs.

Keywords: Aquila fasciata; Blood chemistry; Bonelli’s eagle; Haematology; PCA; Reference values.

Fig. 1

Geographic distribution of the bird origins involved in the study. Nest locations include Andalucía, Madrid, Balearic Islands, Castilla la Mancha, Valencia, and Castilla-León. Breeding centers are situated in Madrid and Vendee (France). Colors depict the number of birds sampled from each region

Fig. 2

Schematic representation of study phases for the selection of reference populations. Outlined Circle: initial dataset; Dashed Diamond: application of exclusion criteria; Parallelogram: curated dataset; Dashed Rectangle: data analysis phase; Solid Rectangle: defined reference populations

Fig. 3

Principal Component Analysis (PCA) factor map displaying blood parameter data from the Bonelli’s eagle nestling reference population (<50 days of age) ($n=117$) (A) PCA factor map illustrates the relationships between biochemical parameters in the total population as coordinates in Dimension 1 and 2 (Dim1 and Dim2). The parameters contributing the most to each dimension are depicted by the longest distance to the coordinate origin and are color-coded with warmer colors (red and orange). Total Proteins and AST exert the highest contributions to Dim 1, while Potassium (K) has the highest contribution to Dim 2. (B) PCA biplot categorized by captivity. Individuals of captive (blue) and wild (yellow) birds are represented as coordinates in the first two-dimensional plane. The confidence ellipse delineates the region containing 95% of samples, indicating that the reference population of nestlings clusters into two distinct populations