The methodology, applicable to most birds of prey and other territorial species, makes it possible to accurately identify the four main demographic processes that cause population changes in a given species: births, deaths, emigration and immigration. This advance will help to have a global view of the demographic processes and to direct conservation strategies on those processes that most affect the viability of the population (increasing births, reducing mortality, retaining specimens that migrate, etc.).

The study, published in the journal Ecological Applications, is signed by the experts Joan Real, Jaime A. Badia-Boher, Antonio Hernández-Matías, Santi Mañosa, Francisco Parés and Diego J. Arévalo-Ayala, from the UB-IRBio Conservation Biology Group, and Josep Maria Bas, from the University of Girona. The study has been supported by the Natural Parks Network of the Barcelona Provincial Council, the Natural Park Network of the Government of Catalonia, the MAVA Foundation and the companies Endesa and Bodegas Miquel Torres, among others.

The Bonelli’s eagle (Aquila fasciata) population in Catalonia — now recovering after many years declining — has been used as a model to implement the new population study methodology after nearly 50 years of scientific monitoring.

Decline and recovery of the Bonelli’s eagle in Catalonia

One of the most outstanding advances in the study of the demography of wild animal populations is the use of the Integrated Population Model (IPM), which has opened the way to study also the immigration process in the studied populations. “These IPMs help to obtain more robust and reliable estimates of population dynamics, since they are formulated so that all sources of data on a wild population (censuses, ringing or tagging data, observations during the breeding season) inform each other”, explains Professor Joan Real, head of the UB’s Department of Evolutionary Biology, Ecology and Environmental Sciences.

“Another advantage of the IPMs is their flexibility. In this study, we have used up to five different types of data, collected over the long term since 1986. This has allowed us to accurately determine demographic processes on births, deaths, emigration and immigration”, says Jaume Badia-Boher, first author of the article, which is part of his doctoral thesis at the UB.

In the case of the Bonelli’s eagle in Catalonia — a species in regression and seriously endangered throughout Europe — the methodology has detailed that the population declined rapidly during the 1980s and 1990s. The population decline of this bird of prey was halted during the 2000s, and since 2010 the population has been recovering progressively. “During the population’s decline, the arrival of immigrant individuals from other populations in the Iberian Peninsula in a better state was probably a relevant factor to prevent the decline from being greater”, says Professor Antonio Hernández-Matías.

Today, with population recovery and youth survival on the rise, “the importance of migrants is lower, while the number of individuals migrating to other populations has increased. This factor may have helped other populations currently threatened in other territories”, adds the expert.

Non-breeding population is also decisive

The results indicate that the floater population — i.e. those that cannot reproduce due to biological or ecological factors and do not have a fixed territory — plays a key regulatory role in the eagle population, an aspect for which there has been very little empirical evidence so far.

When the breeding population declined sharply in the 1980s and 1990s, the floater individuals were not enough to cover the losses, probably also due to high mortality and thus causing the population decline. Conversely, from the 2000s onwards, the floater population recovered, thus helping the breeding population to expand”, explains Professor Santi Mañosa. “We now know that an abundant floater population can help stabilize the breeding population, avoid population declines, and help the species to expand”.

The decrease in mortality of non-breeding juveniles and territorial adults has been decisive for the recovery of the population. “The improvement of the population in Catalonia is also contributing to the viability of nearby populations thanks to the increase in the number of emigrants. Therefore, improving the conservation status of the Catalan population can have a beneficial effect on populations in other territories”, explains expert Jaume A. Badia-Boher.

A recovering albeit fragile population in Catalonia

The results of the study suggest that conservation measures implemented in recent decades — such as the correction of power lines to avoid electrocutions — and perhaps the progressive increase in rabbit populations in areas of juvenile dispersal, may be behind the recovery of the species.

“Coordination between researchers, naturalists, managers, landowners and companies is key to the future of the species. We must continue to detect dangerous nets or power lines and correct them, not only where there are territorial populations, but also in areas where they live and feed non-breeding individuals, as well as improving the habitats in which they live”, says Joan Real.

Despite the recovery of the Bonelli’s eagle in Catalonia, the situation is very fragile and many eagles still die today as a result of accidents with power lines, drowning, gunshots or collisions with infrastructures. “To conserve the species, it is urgent to plan and carry out a more sustainable use of the territory that involves reducing urbanization, limiting the implementation of infrastructures, having a less intensive agriculture and on the other hand, recovering traditional mountain activities favouring biodiversity”, says the team.

In the future, it is necessary to continue scientific monitoring, such as that of the breeding population in their territories, and to gain a better knowledge of the floater population, which is the least known. “Apart from monitoring tasks, it is essential to understand the factors that determine population dynamics, such as the occupation of territories in a changing territorial framework and in a context of a biodiversity crisis that is affecting and extinguishing more and more species”, the experts point out.

A historical note: 50 years of scientific monitoring

This year, it will be 50 years since the first Bonelli’s eagle was tracked in Catalonia. Thanks to long-term studies, it has been possible to detect the warning signs of the conservation status of the populations and thus establish appropriate management and conservation measures. Apart from scientific monitoring, it is also crucial to promote research on basic biology (life cycle, land use, trophic resources, reproduction, survival, etc.) and the factors that condition these aspects. During all these years, the UB-IRBio Conservation Biology Group has promoted both basic and applied research on the species, an activity that has generated different doctoral theses, scientific and informative articles, technical manuals and scientific dissemination actions for the general public.

“This knowledge has made it possible to apply relevant conservation measures on the territory, such as the correction of power lines that had an impact on mortality, the improvement of habitats or the collaboration with private and public land managers. It is worth remembering that public and private organizations, research centres, NGOs, landowners and citizens have collaborated in all these tasks to conserve this endangered species”, concludes Joan Real.
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