The conservation of endangered species is a crucial topic in conservation biology due to the important ecological roles these species play in their respective ecosystems. Top predators are particularly vulnerable to the variation in environmental conditions. The Bonelli's Eagle (Aquila fasciata) is a long-lived bird of prey that fulfills its role as a top predator in Mediterranean ecosystems, but its overall population has declined in the Iberian Peninsula in recent decades. As with other long-lived species, the Bonelli's Eagle reaches its maximum fitness at maturity, highlighting the importance of assessing how intrinsic and extrinsic factors affect population dynamics over time. In this study, we conducted annual surveys of a Bonelli's Eagle subpopulation in the eastern Iberian Peninsula from 2002 to 2021. Our results indicate that the large decline observed in the number of occupied territories, fledged chicks, and successful pairs is not due to negative density-dependent effects but rather influenced by various environmental and demographic factors. These factors include breeders' age, nest-site characteristics, climatic conditions, and previous breeding costs. We found that productivity was higher in territories with high winter precipitation and temperatures, as well as in coastal areas, leading to earlier laying dates. Additionally, the presence of subadult individuals in the breeding pair decreased breeding performance. Interestingly, the variability in the number of fledglings in each territory was driven by previous breeding costs. Prioritizing efforts to mitigate adult mortality and conserve suitable habitats becomes imperative to halt the ongoing decline of this population and also to enhance productivity, ultimately facilitating the restoration of the species to previous population levels.
LAY SUMMARY
The Bonelli's Eagle is a long-lived raptor that serves as a top predator in Mediterranean ecosystems and its population has declined in the Iberian Peninsula in recent decades.
We annually monitored a Bonelli's Eagle subpopulation from 2002 to 2021 in the east of the Iberian Peninsula.
Our findings suggest that the observed decline in the number of occupied territories, fledged chicks, and successful pairs is not caused by factors related to negative density-dependent effects but is instead influenced by a combination of environmental and demographic factors.
Territories occupied by adult breeders and located in milder climatic conditions in coastal areas were those with the highest productivity, leading to earlier laying dates.
The presence of subadult individuals in the breeding pair had a negative effect on reproduction and the variability in the number of fledglings in each territory was driven by previous breeding costs.
Prioritizing efforts to mitigate adult mortality and conserve suitable habitats becomes imperative to halt the ongoing decline of this population and also to enhance productivity.
La conservación de especies en peligro de extinción es crucial en la biología de la conservación debido a los importantes papeles roles ecológicos desempeñados por dichas especies. Los depredadores apicales son particularmente vulnerables a la variación en las condiciones ambientales. El águila perdicera (Aquila fasciata) es un ave rapaz longeva que cumple su función como depredador apical en los ecosistemas mediterráneos y ha disminuido su población en la Península Ibérica en las últimas décadas. Al igual que otras especies longevas, el águila perdicera alcanza su óptimo durante la madurez, lo que resalta la importancia de evaluar cómo los factores intrínsecos y extrínsecos afectan la dinámica de la población a largo plazo. En este estudio, seguimos anualmente una subpoblación de águila perdicera en la parte oriental de la Península Ibérica desde 2002 hasta 2021. Nuestros resultados indican que la gran disminución observada en el número de territorios ocupados, pollos volados y parejas exitosas no se debe a efectos densodependientes, sino más bien a diversos factores ambientales y demográficos. Estos factores incluyen la edad de los reproductores, las características del lugar de nidificación, las condiciones climáticas y el coste de la reproducción en años anteriores. Encontramos que la productividad fue mayor en territorios con elevada precipitación y temperatura invernal, así como en áreas costeras, que presentan fechas de puesta más tempranas. Además, la presencia de individuos subadultos en la pareja reproductora disminuyó el rendimiento reproductivo. Curiosamente, la variabilidad en el número de pollos volados en cada territorio estuvo determinada por el coste de reproducción del año previo. Priorizar los esfuerzos para mitigar la mortalidad de adultos y conservar hábitats adecuados resulta imperativo para detener el declive continuado de esta población y también para mejorar la productividad, facilitando en última instancia la restauración de la especie a tamaños poblacionales anteriores.