Small populations and offspring sex-ratio deviations in eagles

Ferrer, Miguel; Newton, Ian ORCID:; Pandolfi, Massimo. 2009 Small populations and offspring sex-ratio deviations in eagles. Conservation Biology, 23. 1017-1025.

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Stochastic variation of sex ratio has long been appreciated as a potential factor driving small populations to extinction, but it is not the only source of sex-ratio bias in small populations. We examined whether some consequences of sex allocation could affect extinction risk in small populations of size-dimorphic birds such as eagles. We report variations in sex ratio at fledging from a long-term study of a declining population of Spanish Imperial Eagles (Aquila adalberti). Nestling sex-ratio deviation apparently was mediated by age of breeders, whereas territory quality had no obvious effect. Adult–adult pairs produced the same proportion of both sexes in high- or low-density situations, but pairs with at least one member in nonadult plumage class produced more males. As the population declined over a period of years, the proportion of breeders with immature plumage increased; consequently, the proportion of fledgling males increased. However, when population density was high, the proportion of breeders with immature plumage decreased and more female offspring were produced. This relationship between population density, composition of breeder age, and fledgling sex ratios allowed us to make predictions of extinction risk due to nonstochastic deviations of sex ratio in small, declining populations. In the study population, on the basis of the Vortex simulation results, an estimated reduction of 42.5% in predicted mean time to extinction was attributed solely to biased sex ratio.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: CEH Topics & Objectives 2009 - 2012 > Biodiversity
UKCEH and CEH Sections/Science Areas: UKCEH Fellows
ISSN: 0888-8892
Additional Keywords: age of breeding, biased sex ratio, population sex ratio, population decline, population viability, Spanish Imperial Eagle
NORA Subject Terms: Zoology
Date made live: 19 Apr 2011 15:43 +0 (UTC)

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