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What you discover by observing territorial raptor populations during the breeding season

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Birds of prey, like other birds, look for a mate every spring to create a new family. This ecological process of breeding is fascinating, especially when individuals have traveled long distances to choose a breeding site, lay eggs and watch over their nestlings as they grow and, subsequently “leave home”. This explains why we systematically study breeding pairs in the field every spring. Moreover, raptors are useful as ecological indicators since, among other things, they are sensitive to changes in land use, and are highly susceptible to local extinctions. Long-term monitoring of raptor populations, therefore, enables us to identify conservation threats to birds and their habitats, making them an ideal tool for establishing conservation measures. We have published recently an article titled "Long-term population monitoring of a territorial forest raptor species" in the Scientific Data journal.


Study area: “Sierras de Burete, Lavia y Cambrón”, south-eastern Spain. Photo credit: María V. Jiménez-Franco

The data set we have used is based on the census made in a non-invasive, long-term (18 years, 1998-2015) monitoring program of the booted eagle (Hieraaetus pennatus), a territorial forest raptor, in a forested Mediterranean area located in the Natura 2000 Special Protection Area “Sierras de Burete, Lavia y Cambrón”, south-eastern Spain. Although this adventure began in 1996, I am happy to have joined and formed part of the monitoring since 2010. On a typical field day, getting up early was essential to avoid the radiant heat of the midday sun. The first time out, when I heard the first raptor songs of the morning, I didn’t know whether to look through the binoculars to observe individuals more closely or use my camera to take a picture of my first booted eagles in flight. Moreover, it is important to leave the heavy backpack in the car and only carry what the strictly necessary since some slopes in the forest can be rather challenging. Inspecting the pine canopies and finding nest was, to me, like finding a “treasure chest”; even more exciting was, following a short period of observation and detailed examination of the nest, to find evidence of occupancy or reproduction. Sometimes, white down would give away the secret that there were chicks in the nest – now, that really did feel good!


Nestling in the nest (June 2014). Photo credit: María V. Jiménez-Franco

I consider that the way in which our data were collected has special characteristics, involving concern for the ecological patterns of the birds in question, constancy in the fieldwork, discipline in data collection, and, above all, our motivation for scientific study and dissemination of the results. I must not forget to mention the tireless work of a small group of ecologists who helped compile this information, and of course, our gratitude for the financial support received for this research project: Project REN2002-324 01884/GLO, funded by the Spanish Ministry of Education and Science and by FEDER funds and my research fellowships (FPU grant AP2009–2073 from the Spanish Ministry of Education and “Juan de la Cierva-Formación” postdoctoral grant JCI-2015-23508 from the Spanish Ministry of Economy and Competitiveness).

All these "ingredients" enabled us to generate a large cocktail of valuable information concerning our population of booted eagles - the result of meticulous monitoring to obtain information on the nest and territorial occupancy for 163 nests and 72 territories, and the resulting productivity (332 fledglings recorded). We were also able to identify many individuals (48 females and 38 males) and provide details of color polymorphism in booted eagle. This data set covering many years has allowed us to answer many different questions about the populations of territorial birds of prey and what environmental factors influence the choice of territory. We could also contribute to a greater understanding of whether fidelity is conditioned by previous breeding success, the patterns of territorial occupation with other species, and the characteristics of polymorphism and its relationship with productivity.

Fledgling in the nest (July 2014). Photo credit: María V. Jiménez-Franco

Based on the accumulated information, several scientific works have been published on the ecology and conservation of booted eagle (summarized in Table 3 of the article) and developed in three doctoral theses defended by J.E. Martínez, I. Pagán, and M. V. Jiménez-Franco. I have enjoyed the attempt to confirm our hypotheses and compiling a large pool of data, which, I hope, will provide practical guidelines for future studies on the population ecology of raptors during the reproductive period.

Link to the published article: https://doi.org/10.1038/s41597-020-0503-x 

Link to the Dataset: https://doi.org/10.6084/m9.figshare.c.4794411 

María V. Jiménez-Franco

Postdoctoral researcher, University Miguel Hernández of Elche

I am an ecologist focusing on the study of factors that affect site occupancy, distribution, habitat fragmentation, productivity, survival, and interspecific interactions of vertebrate species for application in wildlife conservation and management. My main research tool includes quantitative ecology, analyzing demographic data using classical or Bayesian approaches, mainly hierarchical models, and site occupancy models.