Modelling growth rates and developing an age determination tool for nestling Bonelli's Eagle <i>Aquila fasciata</i>

Abstract Understanding the growth rate of nestlings is essential for explaining developmental strategies and is particularly important for birds of prey. Given that they have lower reproduction rates than other species, the growth rate could be one of the critical factors determining parental fitness and influencing the individual's condition. Despite variability often resulting from various stress conditions, the growth rate is predictable for given ages of nestlings, as highlighted here. This study investigates the growth dynamics of Bonelli's Eagle ( Aquila fasciata ) nestlings in the Republic of Cyprus, focusing on 13 biometric variables measured over time. We employed logistic and generalized additive models to analyse growth trajectories and identify predictors of age. Our findings highlight that nestlings achieve maximum growth rates in body mass and tarsus length early in development, followed by primary feathers and wing length. Conversely, head width, beak dimensions, and talon length exhibit slower and more gradual growth. Precisely describing the rate and pattern of growth of individual body components enabled us to develop a tool for predicting age. We found that wing, tarsus, and middle talon lengths are the most reliable predictors of nestling age. The effectiveness test of our predictive model conducted on an independent data set showed that the age estimation error is 1 day. Thus, our research provides a deep insight into the eagles’ developmental patterns while offering practical tools for field studies and conservation efforts.