Nesting ecology and population genomics of American black ducks in North Carolina

Abstract

North Carolina represents the southernmost extent of the American black duck’s (Anas rubripes) breeding range; however, little is known about their breeding ecology in this region. I located and monitored 140 nesting black ducks over two years (2017–18) to assess nesting productivity and quantify nesting habitat within coastal North Carolina. Specifically, I quantified nest initiation, peak nesting dates, nest success rates, and causes of failure. I further assessed genetic integrity and population structure using high-throughput DNA sequencing methods and identified Anas platyrhynchos × A. rubripes hybrids using ADMIXTURE hybrid filial generation thresholds. Nests were found in brackish marshes (n = 105, 75%) within the Pamlico Sound, and on man-made dredge spoil-islands (n = 35, 25%) along the Outer Banks. The average nest initiation date over the study was April 16 with a peak nesting date of May 7. Apparent nest success rates varied from 31% (2017) to 63% (2018). Predation rates were 42% in 2017 and 28% in 2018. Nest predators included crows (Corvidae), raccoons (Procyon lotor), American mink (Neovison vison), and bald eagles (Haliaeetus leucocephalus).Using MCestimate, I found that the majority (73%) of variability in nest success was modeled best by nest location, vegetation density, maximum vegetation height, and elevation. Nesting black ducks selected high-marsh (NWI code E2EM1P) habitats where nests were located an average of 21.81 m from open water.at mean elevation of 1.36 m. In these habitats visual obstruction readings were 0.5 m with a maximum mean vegetation height of 0.81 m mainly consisting of grass (84.61%). Nesting density was 1 nest per 22 ha. Under the threat of sea level rise, identified selected nesting habitat will increase under the most liberal simulations, allowing for 42 more black duck nests with the study area. Results from my genetic analysis reveal that introgressive hybridization is occurring from the non-western game-farm mallard genome into this population of breeding black ducks. Breeding black duck management efforts in coastal North Carolina should focus on promoting selected nesting habitat, reducing nest predators, conserving lands for future marsh travel, and reducing game-farm mallard contact with black ducks.