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)

Abstract

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.

Both movements and breeding performance are affected by individual experience in the Bonelli's eagle Aquila fasciata

“…We thus expected an effect of weather conditions on breeding performance through the alteration of individual movements. Others studies on other populations showed negative effect of precipitations on Bonelli's eagles breeding performances (López‐Peinado & López‐López, 2023 ). Yet, our results did not show that the breeding performance of Bonelli's eagles was affected by rain or wind, whatever the parameter considered (breeding probability, hatching or fledging success).…”

“…Experienced birds tend to have higher breeding performance (Clutton‐Brock, 1988 ; Daunt et al., 2007 ; Pärt, 1997 ). The effect of age on breeding performance is well known in Bonelli's eagles, with young individuals having lower fecundity than older ones (Carrete et al., 2006 ; López‐Peinado & López‐López, 2023 ; Penteriani et al., 2003 ). We found that the breeding performance of Bonelli's eagles, notably breeding probability and hatching success, was severely reduced when one individual newly recruited, especially the female.…”

“…The Bonelli's eagle reaches sexual maturity between the age of 2 and 5 (Hernández‐Matías et al., 2010 ; López‐Peinado & López‐López, 2023 ). After an erratic phase, individuals settle in a territory in pairs, usually replacing an individual that has disappeared from an existing pair, forcing an individual to leave its territory (take‐over) or colonising a new territory together with a mate (Hernández‐Matías et al., 2011 ).…”

“…Mating takes place from January to March. The female lays one or two eggs (rarely 3, Gil‐Sánchez et al., 2004 ), between February and March, marking the beginning of the incubation phase, which usually lasts 37–41 days (Arroyo et al., 1995 in López‐Peinado & López‐López, 2023 ). During incubation, the female usually stays on the nest, while the male hunts (López‐López et al., 2022 ).…”