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

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 m 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.