Female-biased sex ratio, polygyny, and persistence in the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)

The Condor
By: , and 

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Abstract

Demographic changes in populations, such as skewed sex ratios, are of concern to conservationists, especially in small populations in which stochastic and other events can produce declines leading to extirpation. We documented a decline in one of the few remaining populations of Southwestern Willow Flycatcher (Empidonax traillii extimus) in southern California, USA, which dropped from 40 to 5 adults between 2000 and 2015. Declines were unequal between sexes (94% for males, 82% for females). Adult sex ratios were female-biased in 10 of 16 yr. The proportion of paired males that were polygynous ranged from 0% to 100%, depending on the ratio of females to males in the adult population. Some males paired with up to 5 females simultaneously. We investigated the role of nestling sex ratio in the female-biased adult sex ratio by using genetic techniques to determine sex from blood samples collected from 162 nestlings in 72 nests from 2002 to 2009. Both population-level and within-brood nestling sex ratios were female-biased, and were not influenced by nest order (first or subsequent), parental mating type (monogamous or polygynous), or year. Disproportionately more females than males were recruited into the breeding population, mirroring nestling and fledgling sex ratios. It thus appears that a skewed nestling sex ratio has contributed to a female-biased adult population, which in turn has influenced mating behavior. We propose that the capacity for polygyny, which generally occurs at low levels in Southwestern Willow Flycatchers, has allowed this population to persist through a decline that might otherwise have resulted in extinction.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Female-biased sex ratio, polygyny, and persistence in the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus)
Series title The Condor
DOI 10.1650/CONDOR-16-119.1
Volume 119
Issue 1
Year Published 2017
Language English
Publisher American Ornithological Society
Contributing office(s) Western Ecological Research Center
Description 9 p.
First page 17
Last page 25