Do mallard ducks feature in the diet of stoats in an agricultural landscape?

Research on stoat diet composition in New Zealand has primarily focussed on consumption of indigenous fauna in largely unmodified landscapes. This study used stomach content and stable isotope (δ¹³C and δ¹⁵N) analysis to assess stoat diet in a highly modified agricultural landscape in Southland, New Zealand, focussing on stoat predation of the mallard duck. Stoats were captured in Lochiel, Southland during August–November 2016 and 2017. Stomach content analysis of 26 captured stoats revealed limited stoat predation of mallards (n = 1) and mallard eggs (n = 1). Using liver tissue, stable isotope mixing models suggested that bird eggs on average met between 73 and 85% of stoat metabolic requirements throughout the mallard breeding period. Furthermore, mixing model outputs suggested that bird eggs made up a substantial proportion (77–84%) of stoat assimilated diet early in the mallard breeding period, when mallard eggs are readily available. In contrast, isotope mixing models suggested that mallard ducks/ducklings did not make a large overall contribution to stoat diets (< 3%). This study shows that stoats are an egg predator in the Southland agricultural landscape and mallard eggs may contribute to stoat assimilated diet early in the mallard breeding season before alternative prey items become available.