Nesting ecology of the Hawaiian Duck Anas wyvilliana on northern Kaua‘i, Hawai‘i, USA

Abstract

The nesting ecology of the endangered Hawaiian Duck Anas wyvilliana was studied on Kaua‘i, Hawai‘i, USA between 2012 and 2014. Radio-telemetry and other nest-searching techniques (e.g. foot searches, chain drags) were used to locate and characterise nest sites, and to estimate clutch size and nest success. Nests were found in upland areas associated with a variety of land cover types, including forests, Taro Colocasia esculenta agriculture, managed wetlands, grasslands and scrub/shrub (n = 29). Topographic features associated with nest sites included dikes, upland flats, dry wetland basins and mountain ridges. Distance between nests and water ranged from 0.1–100 m (mean ± s.e. = 26.7 ± 7.3 m; n = 22). Mean height of vegetation at the nests was 103 ± 15 cm, and median overhead cover class was for 66–95% concealment (n = 21). Ground cover within 1 m of nests in forest and scrub/shrub habitat was dominated by Wedelia Sphagneticola trilobata, Uluhe Dicranopteris linearis, Barbas de Indio Andropogon bicornis and California Grass Urochloa mutica (n = 9). Dominant plant species around nests in Taro agriculture primarily included Wedelia, Shortleaf Spikesedge Kyllinga brevifolia and Nodeweed Synedrella nodiflora (n = 8); ground cover in grassland was dominated by Wedelia, California Grass and Guinea Grass Megathyrsus maximus (n = 3). Birds initiated 96% of nests during the eight-month period from October–May inclusive, with 52% occurring during February through April (n = 23). No re-nests were confirmed; however, one radio-tagged female nested three times within a 14-month period, with nest attempts (two successful, one failed) six and eight months apart. Mean clutch size was 5.8 ± 0.4 eggs (range = 4–9; n= 19), and clutch size was higher during the wet season (November–April; 6.1 + 0.4 eggs) than the dry season (May–October; 4.5 ± 0.3 eggs). Nest success was 0.387 (95% CI = 0.150–0.623; n = 20). Causes of nest failure included complete depredation (n = 1), flooding (n = 1), and abandonment due to partial depredation or unknown reasons (n = 6).