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Published on: March 8, 2020
Small populations and offspring sex-ratio deviations in eagles
Miguel Ferrer1, Ian Newton, Massimo Pandolfi
1Department of Biodiversity Conservation, Estación Biológica de Doñana, Consejo Superior de Investigaciones Científicas, Avenida de María Luisa s/n, Pabellón del Perú, Seville, Spain. mferrer@ebd.csic.es
Summary
Sex ratio bias in Spanish Imperial Eagles is influenced by breeder age, not territory quality. This non-stochastic factor significantly increases extinction risk in declining populations.
Area of Science:
- Ornithology
- Conservation Biology
- Population Genetics
Background:
- Sex ratio variation is a known driver of extinction risk in small populations.
- Non-stochastic factors, such as those related to sex allocation, can also bias sex ratios and impact population viability.
- Size-dimorphic birds, like eagles, may experience unique challenges related to sex allocation and extinction risk.
Purpose of the Study:
- To investigate the factors influencing sex ratio variation at fledging in a declining population of Spanish Imperial Eagles (Aquila adalberti).
- To determine if breeder age and territory quality affect nestling sex ratio deviation.
- To assess the impact of non-stochastic sex ratio bias on the extinction risk of this endangered species.
Main Methods:
- Long-term study of a declining Spanish Imperial Eagle population.
- Analysis of nestling sex ratios in relation to breeder age (adult vs. non-adult plumage) and territory quality.
- Population viability analysis using Vortex simulations to estimate extinction risk.
Main Results:
- Nestling sex ratio deviation was mediated by breeder age, with pairs including non-adult plumage breeders producing more males.
- Territory quality did not significantly affect sex ratio.
- As the population declined and the proportion of immature breeders increased, the proportion of male offspring rose, exacerbating extinction risk.
Conclusions:
- Breeder age composition is a critical factor influencing sex ratio dynamics in Spanish Imperial Eagles.
- Non-stochastic sex ratio bias, driven by changes in breeder age structure, significantly increases extinction risk in this declining population.
- A 42.5% reduction in predicted mean time to extinction was attributed to biased sex ratios in the study population.
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