How to recover endangered raptor species: the Spanish imperial eagle as a case study

Rounding up our Endangered Species Day series, Miguel Ferrer demonstrates how scientists and managers can work together and succeed in conservation efforts. Ferrer et al.’s paper, Reintroducing endangered raptors: A case study of supplementary feeding and removal of nestlings from wild populations, was our Editor’s Choice article for issue 55:3

Recently, we published a paper about supplementary feeding of large raptors as a method to increase productivity in certain territories. This allowed us to use this ‘overproduction’ in reintroduction/reinforcement projects without any effect on the stability of donor populations. This approach decreases the budget of a conventional breed-in–captivity based programme by at least eight times. We showed how to produce extra chicks, in a relatively cheap way, to support reintroductions or reinforcement programmes when necessary. This is only the latest development in a long line of research allied to management in restoring an endangered raptor.

Author image 2
Image courtesy of Miguel Ferrer

In 1980, when we started to study the populations of Spanish imperial eagles, only 100 breeding pairs remained in Spain – indeed the world– as the eagles arrived on the Iberian Peninsula almost a million years ago and have never bred outside this area . In Andalusia (south of the Iberian Peninsula), only 22 pairs were breeding in 1980, mainly in the Sierra Morena mountain range and Doñana National Park. Population trends by these dates revealed a marked decline. We began our research then and started fitting radio transmitters to follow juveniles to their dispersal areas. Soon we realised that a major mortality factor was seriously limiting population viability: electrocution on distribution power lines. At the same time, we discovered that the actual area eagles needed to survive in was much more extensive than embraced by protected areas in these years. Furthermore, connectivity between breeding populations was very low due to the disappearance of historical populations in key locations inside a former metapopulation system.

By 1988, around 80% of juvenile eagles had died through electrocution before they reached sexual maturity. We started to work with power companies and environmental administration managers to change the by-law regulations for new power lines as well as to find efficient solutions for the huge network of potentially dangerous power lines in operation. After several years of research and experiments, working with engineers and wildlife managers, we showed we could reduce the electrocutions of eagles in Andalusia by more than 82%, thus providing an opportunity for the recovery of the species.

Following a period of eagle population increases, a new rabbit disease emerged in 1993, decreasing the former density of the main prey of eagles seven-fold. At the same time, due to an increase in the illegal use of poisons (and probably due to wild rabbit population decreases), annual adult mortality of eagles doubled, driving trajectories of populations towards extinction in a short time. Fortunately, we were able to react quickly thanks to the previous development of the ‘early warning signals’ such the proportion of non-adults breeding. Again, working together with wildlife managers, we designed a recovery plan to decrease adult mortality, combined with a strategy to eradicate the illegal use of poisons. The results were fantastic, with some owners of properties where poisons were found suffering preventive suspension of hunting activities due to the human health risk (game species are significant food for people).

Ferrer graph
Population trends during the last 45 years of Spanish imperial eagles in Andalusia. Arrow shows the retrofit of dangerous power poles and the beginning of reintroduction programme in Cadiz.

In a short time, adult eagle mortality returned to normal levels. Additionally, in small populations, we saw significant biases in sex ratio of nestlings. Males were detected, forcing us to implement reinforcement with young females to better-balance the proportion of sexes in the small populations. A few years later, we started a reintroduction programme in one of the strategic locations (Cadiz, Andalusia) to increase the interchange of juveniles among populations and stabilise the future of the species.

Population trajectory for the Spanish imperial eagle is now growing incredibly; over 5% annually, the largest ever recorded. Currently, the population in Andalusia is 110 breeding pairs (compared to 22 in 1980) and the whole eagle population now numbers more than 500 pairs. Clearly, this is a successful conservation story, but for me the take-home lesson is that researchers and wildlife managers working together offer the most efficient means of applying ecological knowledge to solving conservation and management problems. As a researcher, my only opportunity to conduct ‘experiments’ with eagles is to help in the design of conservation plans and to be sure the design is well done. For managers, to get scientists involved in their plans is an extra safeguard. Together, we form the two sides of the coin.

Reintroducing endangered raptors: A case study of supplementary feeding and removal of nestlings from wild populations is the Editor’s Choice for issue 55:3 of Journal of Applied Ecology.  Find out why here.

This post is part of the British Ecological Society journals’ series for Endangered Species Day. Read our other endangered species posts:

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