The endangered Brown Shrike subspecies Lanius cristatus superciliosus has a male-biased sex ratio and only early arriving males are paired

Hiroyoshi Higuchi,Haruka Mizumura,Kohei Kubota

doi:10.1017/s0959270921000216

Hiroyoshi Higuchi, Haruka Mizumura + Show 1 more

Open Access

https://doi.org/10.1017/s0959270921000216

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Abstract

SummaryAlong with many long-distance migrant passerine species in the East Asian-Australasian Flyway, the migratory Brown Shrike Lanius cristatus superciliosus has been sharply declining throughout its breeding range. Its breeding range in Japan shrank by 90.9% between the 1910s and 2010s. In contrast, the closely related but resident Bull-headed Shrike L. bucephalus bucephalus has been gradually declining but is still a common resident in Japan. To better understand the drastic decline of Brown Shrike, we compared the pairing success during three consecutive breeding seasons of these two species. About 60–70% of Brown Shrike males were unpaired, which was much higher than the percentage of unpaired male Bull-headed Shrike (c.0–20%). Brown Shrike males arriving later did not pair because the population’s sex proportion is heavily biased toward males. One of the factors of male-biased population of Brown Shrike may be female-biased mortality in wintering sites, or on the migratory journey, and tracking studies will be required to test this.

Highlights

The East Asian-Australasian Flyway (EAAF), stretching from the Russian tundra to New Zealand’s coasts (BirdLife International 2010), is one of the four globally recognized flyways and encompasses a greater number of threatened migratory bird species than any other flyway (Yong et al 2015)
Where there are many unpaired males in an endangered species, these populations become susceptible to environmental and demographic stochasticity, and genetic degradation (Lande 1993, 1995, Dale 2001), which can be critical for endangered species (e.g. Steifetten and Dale 2006)
It is predicted that if the population sex ratio is biased toward males, only early arriving males will mate with a female

Summary

Introduction

The East Asian-Australasian Flyway (EAAF), stretching from the Russian tundra to New Zealand’s coasts (BirdLife International 2010), is one of the four globally recognized flyways and encompasses a greater number of threatened migratory bird species than any other flyway (Yong et al 2015). Biased sex ratios are common in birds but are more common in globally threatened species than in non-threatened species (Dale 2001, Donald 2007). Where there are many unpaired males in an endangered species, these populations become susceptible to environmental and demographic stochasticity (e.g. fluctuations in climate, habitat conditions, and random variation in sex ratio), and genetic degradation (e.g. inbreeding effects) (Lande 1993, 1995, Dale 2001), which can be critical for endangered species The timing of arrival of long-distance migrants may affect biased sex ratios. It is predicted that if the population sex ratio is biased toward males, only early arriving males will mate with a female

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