Abstract
The adult sex ratio (ASR) is a crucial component of the ecological and evolutionary forces shaping the dynamics of a population. Although in many declining populations ASRs have been reported to be skewed, empirical studies exploring the demographic factors shaping ASRs are still rare. In this study of the socially monogamous and sexually dimorphic Black-tailed Godwit (Limosa limosa limosa), we aim to evaluate the sex ratio of chicks at hatch and the subsequent sex-specific survival differences occurring over 3 subsequent life stages. We found that, at hatch, the sex ratio did not deviate from parity. However, the survival of pre-fledged females was 15–30% lower than that of males and the sex bias in survival was higher in low-quality habitat. Additionally, survival of adult females was almost 5% lower than that of adult males. Because survival rates of males and females did not differ during other life-history stages, the ASR in the population was biased toward males. Because females are larger than males, food limitations during development or sex-specific differences in the duration of development may explain the lower survival of female chicks. Differences among adults are less obvious and suggest previously unknown sex-related selection pressures. Irrespective of the underlying causes, by reducing the available number of females in this socially monogamous species, a male-biased ASR is likely to contribute to the ongoing decline of the Dutch godwit population.
Highlights
The ratio of males to females is a crucial characteristic of any population as it likely affects the competition for mates among individuals and, the population’s mating system, dispersal and migratory behavior, and demographics (Bessa-Gomes et al 2004; Kokko et al 2006; Trochet et al 2013; Lisovski et al 2016; Eberhart-Phillips et al 2018)
In our mark-recapture analysis, the most parsimonious model for resighting probability included an effect of year and sex during the pre-fledging period, a year effect during the post-fledging period, and an interaction term between year and sex for adults
We evaluated the sex ratio at hatch and sex-specific survival rates of Continental Black-tailed Godwits during 3 life-history stages to assess if their adult sex ratio (ASR) was skewed and, if so, when this skew arose
Summary
The ratio of males to females is a crucial characteristic of any population as it likely affects the competition for mates among individuals and, the population’s mating system, dispersal and migratory behavior, and demographics (Bessa-Gomes et al 2004; Kokko et al 2006; Trochet et al 2013; Lisovski et al 2016; Eberhart-Phillips et al 2018). The causes and consequences of variation in the sex ratios of birds have been intensively studied (Weatherhead and Teather 1991; Benito and Gonzales-Solis 2007; Eberhart-Phillips et al 2017). Thereby, studies show that birds are able to mold the sex ratio of their clutches in response to the condition of the mother, their lay date and hatch order, or the quality of the breeding environment (Clout et al 2002; Suarsa et al 2003; Alonso-Alvarez 2006; Dijkstra et al 2010). Skewed initial sex ratios are only one potential determinant of the adult sex ratio (ASR), as sex differences in survival during other life-history stages can contribute to the ASR (Emlen 1997; Weimerskirch et al 2005; Benito and Gonzales-Solis 2007; Eberhart-Phillips et al 2017). Sex-specific reproductive costs during adulthood may cause sex-specific survival rates that potentially introduce a shift in the ASR as well (Tavecchia et al 2001)
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