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Uncovering Skewed Sex Ratios and Polygyny in a Population of Endangered Birds

Released: 2017
WERC Publication Brief: Uncovering Skewed Sex Ratios and Polygyny in a Population of Endangered Birds. Updated March 2017.

This Brief Refers To: Kus, B.E., S.L. Howell and D.A. Wood. 2016. Female-biased sex ratio, polygyny, and persistence in the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus). The Condor 119 (1): 17-25. doi:10.1650/CONDOR-16.119.1

In a study published in The Condor: Ornithological Applications, researchers from the U.S. Geological Survey documented a shift in the sex ratio of a declining population of the Southwestern Willow Flycatcher (Empidonax traillii extimus), an endangered passerine restricted to riparian habitat in the U.S. southwest. Research findings indicated that the population switched from a monogamous to a primarily polygynous mating system.

Demographic changes in small populations, such as changes in sex ratios, are a cause of concern for conservationists because they destabilize populations and raise the risk of population decline, especially in endangered species. Skewed adult sex ratios affect the ability of individuals to find mates, and depending on the mating system, reduce the number of offspring produced per individual and population growth rate.

USGS scientists monitored Southwestern Willow Flycatchers at Marine Corps Base Camp Pendleton in southern California from 2000 to 2015. During this time, they documented a decline from 40 individuals to just five. Declines were unequal between sexes, and females outnumbered males in 10 of the 15 years. As the population became more female-biased, males shifted from monogamy to polygyny, with some males pairing with up to five females simultaneously.

To investigate possible origins of the female-biased adult sex ratio, the researchers collected blood samples from nestlings and used genetic techniques to determine sex. Sex ratios of nestlings were female-biased within individual nests, and in the population as a whole. Moreover, disproportionately more females than males fledged, and later entered the breeding population as adults. It thus appears that a skewed nestling sex ratio has contributed to a female-biased adult population. Future studies into this topic could investigate environmental or other factors that may
 nfluence sex ratios at multiple stages of the life cycle.

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WERC PubBrief 201703 Kus - SW Willow Flycatcher Polygyny.pdf488,126 Bytes

This product is associated with the following projects:
Monitoring Avian Productivity and Survival at Marine Corps Base Camp Pendleton, California

Population Structure and Demography of the Least Bell's Vireo and Southwestern Willow Flycatcher

Management Implications
  • Assessments of demographic factors (such as sex ratio) that influence population dynamics can support effective conservation measures.
  • Flexibility in mating behavior, such as the shift towards polygyny documented in this study, may allow populations to persist through declines that might otherwise result in collapse.
  • Research to elucidate and manage factors that influence sex ratios and population dynamics can assist in recovering endangered species.

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