Male Availability but not Breeding Synchrony Affects Extra-Pair Paternity in a Grassland Specialist: The Grass Wren (Cistothorus platensis)

ARRIETA, RAMIRO SANTIAGO; LEONARDO CAMPAGNA; MAHLER, BETTINA; LLAMBÍAS, PAULO EMILIO

Conferencia; The Northeast Natural History Conference 2021; 2021

Eagle Hill Institute, The Wilson Ornithological Society, the Association of Field Ornithologists

Demographic factors are thought to affect the frequency of extra-pair paternity (EPP) in birds, as the distribution and availability of potential mates in both space and time should determine the rate of encounters between females and males. We examined the influence of breeding density, synchrony, and adult sex ratio (ASR) on the genetic mating system of a south temperate population of Cistothorus platensis (Grass Wren). The Grass Wren is a small, territorial, socially monogamous passerine, broadly distributed across South America. Over three breeding seasons we intensively studied the breeding system of Grass Wrens and genotyped 73 broods (319 nestlings) to estimate the EPP rate and the presence of extra-pair offspring (EPO) in the nest. Our analyses focused on a local demographic scale that considers the spatial interactions of wrens. We calculated the mean distance that individuals moved to obtain extra-pair fertilizations (130.4±0.1 m). We then defined a radius of 130 m around each nest containing a genotyped brood and calculated within this area the local breeding density, synchrony, and ASR. We found that the EPP rate increases with the number of males (higher local ASR). However, the local ASR did not affected the presence of EPO in the nest. Neither the local breeding density nor the local breeding synchrony influenced the EPP rate or the presence of EPO in the nest. Our results suggest that extra-pair behavior in Grass Wrens was driven by male availability while the simultaneous fertility of females did not affect extra-pair fertilizations. Breeding synchrony may increase EPP rates if the simultaneous display of males allows females to more accurately compare potential mates. However, year-round territoriality of Grass Wrens may allow females to assess social and extra-pair mates throughout the year. Setting a local demographic approach that effectively reflects the social interactions among individuals is essential to evaluate the impact of demographic factors on extra-pair behavior.