Construção de múltiplos ninhos como estratégia reprodutiva para o Garibaldi, Chrysomus ruficapillus (Aves, Icteridae)

Abstract

The main function of bird's nests is to provide protection for eggs and young, and due the high risks of predation, nests are under intense natural selection. Nest construction is time and energetically costing, which limits the number of nests constructed in a breeding season. For this reason, intense and elaborated constructions can provide cues about male fitness, influencing in sexual selection. However, a small number of species has developed the construction of multiple nests in the same breeding territory as a reproductive strategy, and some of hypotheses have been proposed to explain such behavior. Although some o these hypotheses are not mutually-excludent, most works have addressed only one or two of these hypotheses. The Chestnut-capped Blackbird, Chrysomus ruficapilus, belongs to the family Icteridae and it is widely distributed in South America, being a common element in floodable areas. Previous works have reported the presence of non-breeding nests for this species and they were treated as abandoned nests, but the real functions of these extra-nests were never investigated. Here we addressed the function of the extra-nests in C. ruficapillus by testing the following hypotheses: (1) old nests hypothesis; (2) inefficient concealment hypothesis, which predicts that extra-nests are nests that were abandoned because they were constructed in highly exposed sites; (3) predation avoidance hypothesis, and the (4) female attraction hypothesis. To achieve this purpose we carried out periodical visits to the study area, between October 2017 to April 2018, and marked males within their territories and their nests were monitored. To evaluate the above hypotheses we tested for correlations between numbers of nests in the territories and: numbers of clutch initiations; numbers of fledglings; probability of males being chosen by females, and nest survival. Further, a number of environmental covariates that could influence in these parameters were controlled. Nests never lasted across breeding seasons, eliminating the old nest hypothesis. Used and non-used nests did not differ in relation to vegetation density parameters, providing low support to the inefficient concealment hypothesis. Males constructed up to 14 nests in a season, and had up to six clutch initiations. The female attraction hypothesis was corroborated, as the number of nests was correlated to the number of clutch initiations and number of fledglings. However, females have not actively selected for males with the higher numbers of nests in their territories, and nest survival was also non-correlated to the number of extra nests. Then, our results give support to the idea that males build multiple nests to increase their reproductive fitness.