Resource Use Overlap by Sympatric Wintering American Black Ducks and Mallards in Tennessee

American black ducks Anas rubripes are declining in traditionally important wintering areas in the south-central Mississippi flyway.  Understanding resource exploitation by black ducks and morphologically similar and co-existing mallards A. platyrhynchos during winter may provide insight into competitive interactions which could exacerbate these declines.  We radiomarked female black ducks and mallards at the Tennessee National Wildlife Refuge, Tennessee USA, during winter 2011–2012.  We hypothesized that resource partitioning may occur to avoid or lessen interspecific competition between the two species and examined this in the context of home range overlap, resource selection and proportional resource use, niche breadth, and inter- and intraspecific resource overlap.  Black duck and mallard home ranges were similar in area, with black duck individual home ranges ranging from 15 to 77 km2 and mallards from 21 to 72 km2 in size; 93% of the total area used by each species overlapped. Black ducks selected emergent wetlands 1.2x more than mallards, and mallards selected cultivated lands 2x more than black ducks.  However, there were only minor differences between species in their proportional use of landcover types, and there were no temporal differences in resource selection at the diurnal or seasonal scale.  Additionally, while black ducks and mallards had moderate and small niche breadths (niche breadth = 0.47 and 0.34, respectively), the two species had a substantial degree of inter- and intraspecific resource overlap.  Our results indicate that these species do not competitively exclude each other spatiotemporally.  Further research is needed to understand fine-scale interactions between these species and other factors which may be impacting black duck declines in the south-central Mississippi flyway, such as effects occurring in the breeding grounds or the potential influences of climate on northern shifts in duck distribution.