Male-skewed adult sex ratio, survival, mating opportunity and annual productivity in the Snowy Plover Charadrius alexandrinus

Authors


Corresponding author.
Email: lstenzel@prbo.org

Abstract

Sex differences in adult mortality may be responsible for male-skewed adult sex ratios and male-skewed parental care in some birds. Because a surplus of breeding males has been reported in serially polyandrous populations of Snowy Plover Charadrius alexandrinus, we examined sex ratio, early-season nesting opportunities, adult survival and annual reproductive success of a Snowy Plover population at Monterey Bay, California. We tested the hypotheses that male adult survival was greater than female survival and that a sex difference in adult survival led to a skewed adult sex ratio, different mating opportunities and different annual productivity between the sexes. Virtually all females left chicks from their first broods to the care of the male and re-nested with a new mate. As a result, females had time to parent three successful nesting attempts during the lengthy breeding season, whereas males had time for only two successful attempts. Among years, the median population of nesting Plovers was 96 males and 84 females (median difference = 9), resulting in one extra male per eight pairs. The number of potential breeders without mates during the early nesting period each year was higher in males than in females. Adult male survival (0.734 ± 0.028 se) was higher than female survival (0.693 ± 0.030 se) in top-ranked models. Annually, females parented more successful clutches and fledged more chicks than their first mates of the season. Our results suggest that in C. alexandrinus a sex difference in adult survival results in a male-skewed sex ratio, which creates more nesting opportunities and greater annual productivity for females than for males.

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