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The cheaper sex and extinction…for Imperial Eagles

Photo: Carlos Sanz
Photo: Carlos Sanz

Manipulating egg quality and brood mates  is a strategy that is not uncommon for birds.  In fact, this is an important tactic in the survival of species when food is scarce and territories are poor.  The idea is that deteriorating environmental conditions will drive parents to invest more resources in the sex that maximizes their fitness, also called the cheaper sex.  And when environmental conditions once again become favorable, the sex ratio of males to females will eventually return to a state of equilibrium.

But when genetic predisposition for altering the sex ratio of a population is combined with an age bias and a small existing population, extinction risk creeps into the picture.  And a species that fits this bill is the Spanish Imperial Eagle.

The very rare Spanish Imperial Eagle has a population hovering around 200 breeding pairs (split into three subpopulations with relatively little interchange), which is a phenomenal recovery from the only 30 pairs that remained in the 1960s.  It is a non-migratory raptor that can approach a weight of 8 pounds, has a low reproductive rate, a lifespan of approximately 22 years in the wild, and of special note an immaturity period of 4-5 years.  According to a study in the Journal of Conservation Biology, since breeding age is a factor of density in this case, when the population of Imperial Eagles declines below capacity level the number of immature breeders increases.  And with a species harboring an age bias, an increase in immature breeding individuals resulted in a higher production of male offspring, which just so happens to be the cheaper sex for Imperial Eagles.

Photo: Antonio Lucio Carrasco Gómez
Photo: Antonio Lucio Carrasco Gómez

“Our findings show that fledgling sex ratio could be biased at the population level, and that bias toward the cheaper sex (males for Imperial Eagles) could be related to parental age. Because the mean age of breeders is density-dependent in Spanish Imperial Eagles, sex ratio ultimately may be related to population density.”

Thus, Spanish Imperial Eagles are finding themselves potentially embroiled in a vicious circle as the relatively low numbers in each of the three subpopulations are causing a decrease in the first breeding age and driving increases in male offspring.  Both of which are increasing the extinction risk as sex ratios are skewed from equilibrium. The authors found that when the number of males reached a level of 72% of the population (as opposed to 50% at equilibrium), the average time to extinction was reduced by almost 43%.

FERRER, M., NEWTON, I., & PANDOLFI, M. (2009). Small Populations and Offspring Sex-Ratio Deviations in Eagles Conservation Biology, 23 (4), 1017-1025 DOI: 10.1111/j.1523-1739.2009.01215.x

Scott Artis
Scott serves as Director of Development & Communications for Audubon Canyon Ranch (focusing on preservation, education and conservation science) and has almost fifteen years of experience spanning for-profit and nonprofit sectors in biotech, wildlife conservation and management, communications, and philanthropy. In addition to a strong track record in organizational growth and leadership, he is the founder of Urban Bird Foundation and Burrowing Owl Conservation Network, and presided over ECHO Fund, a coastal protection and restoration organization, as President for four years. Scott holds an M.A. in Environmental Studies with a concentration in Sustainable Development and Policy, degrees in Micro & Molecular Biology and Environmental Sciences, and has complemented his studies with a Master's certificate in Environmental Resource Management.


  1. It appears that this study doesn’t bode well for the Spanish Imperial Eagles or most other endangered species. I guess this would be an argument for human management of species in peril.

    Am I understanding this correctly that the “cheaper sex” is the one that takes less food and territory depending on species? Wouldn’t that almost always be the male of the species being that females lay the eggs creating more mouths to feed?

    • Hey Larry, actually the ‘cheaper sex’ is the one that is the least costly to rear. For imperial eagles it is believed that the females are expensive to rear because they grow more rapidly to a larger size as fledgings, and thus
      nestling females may have greater food needs than males.

      However, in Red-winged blackbirds for example, the males are the more expensive sex as they grow faster to a larger size and are more prone to starvation….which means young, inexperienced females fledge more daughters.


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