Breeding Ecology and Productivity of Mallards and Mallard-grey Duck Hybrids in New Zealand

Abstract

Introduced primarily for sport hunting, mallards (Anas platyrhynchos) became widely established in New Zealand (NZ) following the release of 25,000 individuals during 1940–1960. Hybridisation and introgression between mallards and the native grey duck (A. superciliosa) has been extensive, and today, mallards and mallard-grey duck hybrids (hereafter mallards) are the predominant game bird in NZ. Perceived declines in mallard abundance in some regions have prompted NZ Fish and Game Council to initiate research to better understand causes of population change. During 2014–2015, I collected data from 304 radiomarked female mallards, 491 nests, and 190 broods from 2 study sites in NZ (Southland and Waikato) to answer essential questions about breeding season vital rates and habitat requirements, and to determine factors important in affecting population growth rates. Breeding incidence averaged 0.91 (SE = 0.03), renesting propensity following failure of nests or broods was 0.50 (SE = 0.03), egg hatchability of successful nests was 0.93 (SE = 0.01), partial depredation occurred in 0.16 of nests (SE = 0.16), and daily nest survival was 0.9789 (SE = 0.17). Cumulative nest survival ranged from 0.22 for nests along drainage ditches in Waikato to 0.61 when they were located along roadsides in Southland. Mean daily brood survival was 0.9816 (SE = 0.003) and cumulative survival ranged from 0.16 for second-year (SY) females in Waikato to 0.30 for after-second year females (ASY) in Southland. Female breeding season survival averaged 0.79 (SE = 0.06) and post-fledging survival was 0.51 (SE = 0.008). Older females had higher breeding effort and reproductive success; they nested earlier, laid larger clutches, hatched more eggs per nest, and fledged more ducklings. Predicted fecundity suggested ASY and SY females recruited 0.25 and 0.36 female offspring into the breeding population, respectively. Model-predicted population growth rates suggested an annual decrease of 0.16 per year. Sensitivity analyses indicated that duckling survival, particularly of older females, was the most influential factor regulating growth of mallard populations, followed by breeding survival of ASY females and duckling survival of SY females. Management initiatives that focus on improving survival of ducklings and females will have the greatest potential to increase duck production.