Population Dynamics of Threatened Piping Plovers on the Niobrara River, Nebraska
Prairie rivers of the Great Plains, USA, provide important habitat for the federally threatened piping plover (“plover”, Charadrius melodus). Plovers nest on open to sparsely vegetated river sandbars, and their demographic rates are closely linked to habitat availability and quality, as well as river flow. The Niobrara River in northern Nebraska has supported 22–41% of the state’s plovers since species listing in 1986, but the population and habitat are relatively understudied, and both have declined since 2010. The objectives of this study were to understand plover demography, habitat, and the role of the Niobrara in the regional plover population.
Periods of high river flow promote creation and maintenance of suitable sandbar nesting habitat, but increased river flow during the plover breeding season can decrease nest and chick survival. We estimated the effect of daily peak river flow on survival rates of 115 nests and 66 chicks on the Niobrara River, 2010–2016, using logistic exposure and Cormack-Jolly-Seber models, respectively. We monitored 1,874 banded hatch-year and adult birds across the regional population (Niobrara River, Lewis and Clark Lake, Gavins Point Reach segment of the Missouri River), and used multi-state mark-recapture models to estimate survival and inter-annual dispersal probabilities among sites relative to habitat availability. We developed land cover datasets from high-resolution aerial imagery to quantify suitable habitat and compare the relative effects of habitat characteristics on nest-site selection and nest success for a subset of years (2010, 2012, 2014, and 2016) using logistic regression models. We included data from a sympatric nester with similar nesting habitat needs, the interior least tern (“tern”, Sternula antillarum athalassos), to improve precision of our models. We compared 63 plover and 92 tern nests to 292 random unused points, and 73 successful (hatched ≥1 egg) to 79 failed nests.
Low nest and chick survival and high emigration from the Niobrara appear to be important factors contributing to population decline. Daily nest and chick survival were negatively related to river flow. Nest-site selection was based primarily on distance to the river bank (i.e., the nearest potential source of predators), yet flooding (eggs submerged or washed out of the nest bowl during increased river flow) caused at least as many nest failures as predation. Nests predominantly were surrounded by dry sand habitat, indicating some degree of flood avoidance, but were no farther from water than random, and drier nest sites were no less likely to fail. Dispersal occurred throughout the regional population, but plovers were more likely to leave the Niobrara than to enter it. Expansive flood-created sandbars on the Missouri River, concurrent with a trend towards more vegetated and saturated habitat on the Niobrara, may have drawn birds from the Niobrara population, especially those that dispersed to the Niobrara during sustained Missouri River flooding 2010–2011.
The outsized negative effect of flooding on nest success, the lack of protection afforded by dry sand nest sites, and selection for nesting habitat based more strongly on predator avoidance than flood avoidance suggest that plovers may have face more frequent and intense levels of breeding season flooding than is typical. Identifying and promoting the processes that contribute to creation and maintenance of high-elevation sandbars on the Niobrara is an important next step towards effective management of nesting birds.