Adult sex ratios and their implications for cooperative breeding in birds

Abstract

Cooperative breeding is a form of breeding system where in addition to a core breeding pair, one or more usually non-breeding individuals provide offspring care. Cooperative breeding is widespread in birds, but its origin and maintenance in contemporary populations are debated. Although deviations in adult sex ratio (ASR, the proportion of males in the adult population) have been hypothesized to influence the occurrence of cooperative breeding because of the resulting surplus of one sex and limited availability of breeding partners, this hypothesis has not been tested across a wide range of taxa. By using data from 188 bird species and phylogenetically controlled analyses, we show that cooperatively breeding species have more male-biased ASRs than non-cooperative species. Importantly, ASR predicts helper sex ratio: in species with more male-biased ASR, helper sex ratio is also more male biased. We also show that offspring sex ratios do not predict ASRs, so that the skewed ASRs emerge during the period when individuals aim to obtain a breeding position or later during adulthood. In line with this result, we found that ASR (among both cooperatively and non-cooperatively breeding species) is inversely related to sex bias in dispersal distance, suggesting that the cost of dispersal is more severe for the further-dispersing sex. As females usually disperse further in birds, this explains the generally male-biased ASR, and in combination with benefits of philopatry for males, this probably explains why ASR is more biased in cooperatively breeding species. Taken together, our results suggest that a sex bias in helping in cooperatively breeding species relates to biased ASRs. We propose that this relationship is driven by sex-specific costs and benefits of dispersal and helping, as well as other demographic factors. Future phylogenetic comparative and experimental work is needed to establish how this relationship emerges.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

Sex‐biased survival contributes to population decline in a long‐lived seabird, the Magellanic Penguin

“…We therefore assumed that fledgling sex ratio was 0.5 in our population. It is important to note that ASR shows low sensitivity to hatching sex ratio in seabirds (Eberhart-Phillips et al 2018) and neither hatching nor fledging sex ratios predict ASR among birds (Sz ekely et al 2014a, Komdeur et al 2017.…”

“…Variation in seabird movement patterns can drive local population dynamics more than adult survival rates or recruitment (Szostek et al 2014) and rates of dispersal can vary with environmental conditions (Dugger et al 2010). Furthermore, female birds may be more likely to emigrate than males, particularly in declining populations (Morrison et al 2016), and sex-biased dispersal can lead to skewed ASRs (Becker et al 2008, Pipoly et al 2015, Komdeur et al 2017).…”

Adult sex ratios and reproductive strategies: a critical re-examination of sex differences in human and animal societies

“…Does ASR also predict the occurrence of cooperative breeding and helper sex ratio? Based on the results of a comparative study across nearly 200 species of birds, Komdeur et al [57] report that cooperatively breeding species have significantly more male-biased ASRs than non-cooperative breeding species. Moreover, the ASR predicts the sex ratio of helpers.…”

How Many Females Are There? Cheating and Female Dispersion Can Explain Mating Behavior Evolution

“…Although the model was conceived with mammals in mind, there is nothing in it that makes it applicable exclusively to mammals. For example, our model may also explain the high prevalence of social monogamy in birds (29), since reported OSRs of bird populations are usually male-biased (30) and females are more dispersed, which has been implicated as driving cooperative breeding (31), in a situation where our model consistently predicts monogamy.…”