Unexpected diversity of alternative reproductive tactics revealed in Water Rails ( <i>Rallus aquaticus</i> )

Monogamy in birds was long considered a general mating system until molecular studies provided evidence of alternative reproductive strategies (ARS), which are now assumed to be present in ~90% of bird species. These ARS or tactics (ART), as we consider them in this study, include extra‐pair copulation, which can lead to extra‐pair paternity or extra‐pair maternity, also referred to as quasi‐parasitism, and conspecific brood parasitism. Within the family Rallidae, where several species exhibit high rates of ART, the social system of Water Rails Rallus aquaticus has been poorly studied because of their secretive behaviour and occurrence in difficult‐to‐access habitats. In this study, we analysed microsatellites of embryos from 52 eggs collected across six Water Rail nests to infer their parents' reproductive strategies. Our study showed that the species is not genetically monogamous, revealing a complex network of ART. Even quasi‐parasitism, which has been detected in only a few avian species and is poorly understood, was observed in one nest, where the resident male sired one egg each with two other females. Additionally, given recent findings that some female birds lay eggs with a relatively consistent and distinct appearance, we tested the use of photographic data as a potential non‐invasive method for identifying maternity and exploring ART. Although our investigations into the shape, colour and pattern of eggs showed individuality in the morphology of eggs laid by different females, the level of discrimination was not high enough to allow non‐invasive identification of ART in the field using egg morphology alone. Nevertheless, our study confirmed an unexpected high rate of ART in Water Rails.