“Bug-eggs” enable parental sampling and characterization of the genetic mating system of Cattle Egrets

Breeding in dense colonies during a limited time-period, characteristics of many waterbirds – herons, egrets and spoonbills – may have promoted the evolution of non-monogamous reproductive tactics. Yet, to date, only a handful of studies have investigated the genetic mating systems of waterbirds, mostly because the difficulty of sampling elusive candidate parents has hindered the application of conventional DNA-based parentage tests. Here, we characterize the genetic mating system of Cattle Egrets Bubulcus ibis breeding in a natural colony located on trees. Using, for the first time, the Neotropical bug Panstrongylus megistus contained in fake fiber-glass eggs, we collected blood from incubating males and females in 31 nests. We drew blood from the nestlings (n = 89) at those nests, and genotyped all samples at 14 microsatellites. In line with previous behavioral observations, we found evidence supporting a non-monogamous genetic mating system in Cattle Egrets. The parentage allocation method inferred extra-pair paternity (EPP) in 62% of nests and conspecific brood parasitism (CBP) in 64% of nests, while the kinship classification of nestlings in broods disregarding parental information inferred EPP in 50% and CBP in 43% of nests. These results indicate that inferences about the genetic mating system made in the absence of parental information could underestimate the ‘true’ rates of alternative reproductive tactics occurring in nature. We expand knowledge on the genetic mating system of colonial waterbirds, highlight the importance of including parental samples in these analyses, and contribute new information to guide management strategies of the highly invasive Cattle Egret.