Morphometrics and viability rates support a case of hybrid vigour among New Zealand’s Mallard and Pārera (Grey Duck)

The introduction of Mallards (Anas platyrhynchos) into New Zealand (NZ) established a self-sustaining feral population, with many hybrids arising through ongoing interbreeding with endemic Grey Ducks (Anas superciliosa superciliosa; Pārera). We investigated whether NZ Mallards and verified contemporary Pārera × NZ Mallard hybrids exhibit hybrid vigour by comparing morphometrics (body mass, wing chord and culmen length) and viability metrics (fecundity and nest success) against Pārera. Pārera were generally smaller than NZ Mallards, while hybrids were intermediate, with backcrossed individuals tending towards the size of their respective parent species. Importantly, a binomial generalised linear model of daily nest survival revealed that hybrid females outperform NZ Mallards, with female age, body size and condition also significantly influencing survival. Despite clear morphological differences among genetic groups, these results indicate that hybrids gain a measurable advantage in nest viability. Given that NZ Mallards and contemporary hybrids are at least as viable as Pārera, and that hybrids show higher daily nest survival than Mallards, our findings are consistent with expectations of hybrid heterosis. In the Anthropocene, understanding the adaptive potential of hybrids is critical as anthropogenic hybridisation rises. For Pārera, hybridisation with Mallards represents one of the multiple existential threats, alongside habitat loss, overharvest and exotic predators. However, the elevated survival of hybrid populations suggests that if Pārera were lost, hybrids could successfully occupy their ecological niche, with implications for conservation management of endemic waterfowl.