What the direction of matings can tell us of hybridisation mechanisms in ducks

Interspecific hybridisation is common amongst birds and is associated with diverse costs and benefits. Several mechanisms are thought to promote hybridisation, including heterospecific mate choice, species recognition errors and interspecific forced copulations. Discriminating between these mechanisms is often problematic due to difficulties in observing interspecies copulations in the wild. The directionality of hybridisation may, however, provide some insights into which mechanisms are operating. For example, matings may be unidirectional when females prefer to mate with heterospecific males with a specific trait, such as higher dominance or a supernormal stimulus. In contrast, bidirectional hybridisation is more likely when other mechanisms, such as heterospecific imprinting as a result of brood mixing, are operating. We here characterise the direction of matings between Grey Teal (Anas gracilis)×Pacific Black Duck (A. superciliosa) hybrids. By sequencing a fragment of the maternally inherited mitochondrial DNA, we show that hybridisation occurs bidirectionally between these species. This suggests that a female preference does not exist in both species for a specific male trait of only one species. We suggest that errors in species recognition are improbable and that heterospecific mate choice, possibly due to brood mixing, may more likely promote hybridisation between Grey Teals and Pacific Black Ducks.