Breeders' age, nest-site characteristics and climatic conditions but not density-dependent effects determine Bonelli's Eagle breeding performance: A long-term study (2002-2021)

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. center dot 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.center dot We annually monitored a Bonelli's Eagle subpopulation from 2002 to 2021 in the east of the Iberian Peninsula.center dot 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.center dot 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.center dot 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.center dot 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 conservacion de especies en peligro de extincion es crucial en la biologia de la conservacion debido a los importantes papeles roles ecologicos desempenados por dichas especies. Los depredadores apicales son particularmente vulnerables a la variacion en las condiciones ambientales. El aguila perdicera (Aquila fasciata) es un ave rapaz longeva que cumple su funcion como depredador apical en los ecosistemas mediterraneos y ha disminuido su poblacion en la Peninsula Iberica en las ultimas decadas. Al igual que otras especies longevas, el aguila perdicera alcanza su optimo durante la madurez, lo que resalta la importancia de evaluar como los factores intrinsecos y extrinsecos afectan la dinamica de la poblacion a largo plazo. En este estudio, seguimos anualmente una subpoblacion de aguila perdicera en la parte oriental de la Peninsula Iberica desde 2002 hasta 2021. Nuestros resultados indican que la gran disminucion observada en el numero de territorios ocupados, pollos volados y parejas exitosas no se debe a efectos densodependientes, sino mas bien a diversos factores ambientales y demograficos. Estos factores incluyen la edad de los reproductores, las caracteristicas del lugar de nidificacion, las condiciones climaticas y el coste de la reproduccion en anos anteriores. Encontramos que la productividad fue mayor en territorios con elevada precipitacion y temperatura invernal, asi como en areas costeras, que presentan fechas de puesta mas tempranas. Ademas, la presencia de individuos subadultos en la pareja reproductora disminuyo el rendimiento reproductivo. Curiosamente, la variabilidad en el numero de pollos volados en cada territorio estuvo determinada por el coste de reproduccion del ano previo. Priorizar los esfuerzos para mitigar la mortalidad de adultos y conservar habitats adecuados resulta imperativo para detener el declive continuado de esta poblacion y tambien para mejorar la productividad, facilitando en ultima instancia la restauracion de la especie a tamanos poblacionales anteriores.

Automated summary

The conservation of endangered species is crucial in conservation biology due to their important ecological roles. This study monitored a subpopulation of Bonelli's Eagle in the eastern Iberian Peninsula for 19 years and found that the decline in population was influenced by a combination of environmental and demographic factors.