Duckling survival and habitat selection of brood-rearing mallard (Anas platyrhynchos) females in Southland, New Zealand
Southland has been long recognized as a productive area for mallard ducks (Anas platyrhynchos) in New Zealand. Populations have declined in recent years, and these declines have coincided with an increased intensity of conversion of sheep and deer farms into pastoral dairy farms. Sheep and deer farming comprise many short-grass pastures during the waterfowl breeding period, whereas dairy cattle are typically rotated around pastures in a manner that results in many long-grass pastures that are intensively defoliated by grazing every few weeks. Some critics have blamed dairying for the decreases in mallard productivity. In 2014, I investigated environmental factors and female characteristics affecting mallard duckling survival, including: pasture type (long or short grass), percent dense nesting cover within a buffer of the areas used by broods, presence of ephemeral water, distance to the nearest permanent water source, distance to the nearest anthropogenic structure, brood size, egg volume, female age, date of hatch, precipitation, duckling age, and average distance moved from the nest site. I monitored 438 ducklings from 50 radio-marked females to 30 days post hatch. I modeled ragged telemetry data using the nest survival module in Program MARK and evaluated model fit using Akaike’s Information Criterion adjusted for small sample size and overdispersion (QAICc). Duckling survival was unaffected by pasture type, but increased with duckling age (β = 0.05, 85% CI = 0.02 – 0.08), the presence of ephemeral water (β = 0.58, 85% CI = 0.15 – 1.01), and with greater distance from the nearest anthropogenic structure (β = 0.28, 85% CI = 0.02 – 0.54). Survival was lower for broods of second year (SY) females than for broods of after second year (ASY) females (β = -0.52, 85% CI = -0.90 – -0.13), in areas with more dense nesting cover (β = -0.37, 85% CI = -0.60 – -0.15), and when ducklings moved, on average, greater distances (β = -0.33, 85% CI = -0.56 – -0.10). Cumulative 30 day duckling survival ranged from 0.11 (85% CI = 0.07 – 0.15) for ducklings of SY females without ephemeral water present to 0.46 (85% CI = 0.41 – 0.51) for ducklings of ASY females with ephemeral water present. Compositional analyses indicated females selected for dense nesting cover at both the landscape scale (30 km2) and within habitat corridors used by their broods. A resource selection function revealed brood-rearing females preferred dairy pastures within areas used by their broods, areas further from anthropogenic sources, and dense nesting cover. My results show that duckling survival is low in Southland relative to estimates using similar methods from North America. Further, dense nesting cover is selected for by brood-rearing females, but translated into lower duckling survival. Narrow, linear, small patches of dense nesting cover could support a greater abundance of predators, or enable greater foraging efficiency of predators. Mallard females might be selecting habitat to maximise another aspect of their life history (e.g., adult female survival, nest success) at the expense of duckling survival.
Advisor: Seddon, Philip
Degree Name: Master of Science
Degree Discipline: Zoology
Publisher: University of Otago
Keywords: Anas platyrhynchos, duckling survival, ephemeral water, habitat, mallard, New Zealand.
Research Type: Thesis