Local male breeding density affects extra‐pair paternity in a south temperate population of grass wrensCistothorus platensis

Abstract

Demographic factors can affect the frequency of extra‐pair paternity (EPP) in birds, as the distribution and availability of potential mates in both space and time influence the rate of encounters between females and males. Over three breeding seasons, we intensively studied the breeding system of a south temperate population of grass wrens Cistothorus platensis by genotyping 73 broods (319 nestlings) and estimating EPP rates for those broods. Using five different radii (80, 160, 240, 320 and 400 m) around each nest with assigned paternity, we examined the effects of local breeding synchrony, male breeding density and adult sex ratio (ASR) on the EPP rate. The majority of extra‐pair offspring (~80%) were sired by neighboring males. Neither local breeding synchrony nor ASR consistently explained the EPP rate variation as their effects were only statistically significant within 320 and 400 m. However, the EPP rate increased as the local male breeding density increased within every radius category, strongly suggesting that neighboring male abundance might play an important role in the extra‐pair mating behavior in this species. Our study also highlights the relevance of using a local scale approach when studying mating behavior.

Female Grass Wrens adjust brooding effort according to ambient temperature, male feeding contribution and nest placement

“…2018, Zarco & Llambías 2018, Fujikawa & Tubelis 2020) with moderate rates of extra‐pair fertilizations (Arrieta et al . 2022). Nests are dome‐shaped structures, constructed with grass stems, leaves and inflorescences (Zarco & Llambías 2018, Llambías et al .…”

“…The Grass Wren is a small (c. 9 g), insectivorous, grassland specialist, broadly distributed throughout South America (Zarco & Llambías 2018, Remsen Jr. et al 2021. Grass Wrens are year-round territorial, predominantly socially monogamous (Llambías et al 2018, Zarco & Llambías 2018, Fujikawa & Tubelis 2020 with moderate rates of extra-pair fertilizations (Arrieta et al 2022). Nests are dome-shaped structures, constructed with grass stems, leaves and inflorescences (Zarco & Llambías 2018, Llambías et al 2020.…”

Sex‐specific recruitment rates contribute to male‐biased sex ratio in Adélie penguins

“…Stochastic variation of sex ratios in wild bird populations on decadal time scales has been less well-studied, but can cause mating system shifts (Kus et al, 2017), influence mate selection behavior (Madden & Whiteside, 2013), and the prevalence of extra-pair mating in monogamous systems (Arrieta et al, 2022;Grant & Grant, 2019).…”

“…Stochastic variation of sex ratios in wild bird populations on decadal time scales has been less well‐studied, but can cause mating system shifts (Kus et al., 2017 ), influence mate selection behavior (Madden & Whiteside, 2013 ), and the prevalence of extra‐pair mating in monogamous systems (Arrieta et al., 2022 ; Grant & Grant, 2019 ). Sex ratio variation can also increase mate‐competition (e.g., Ewen et al., 2011 ), and decrease the duration of long‐ and short‐term pair bonds (Eberhart‐Phillips et al., 2018 ; Maness & Anderson, 2007 ; Pilastro et al., 2001 ).…”

A possible case of offspring sex manipulation as result of a biased adult sex ratio

“…In this study, we report a case of offspring sex ratio manipulation in a grassland passerine, the grass wren Cistothorus platensis . The grass wren is an excellent model to study offspring sex ratio manipulation as males have higher reproductive value than females due to the potential for social polygyny and extra-pair paternity 30 , 31 . Hence, it is expected that males would exhibit higher reproductive variance than females, and that males in better body condition would have higher reproductive value than females.…”