Diving-duck Productivity: Effects of Predator Management on Nest Success and Investigating Sightability-adjusted Brood-pair Ratios

Abstract

Nest success, a major component of productivity, is often used as the metric to measure the effectiveness of various management efforts aimed at increasing waterfowl productivity. Although numerous studies have proven predator reduction increases nest success for upland nesting waterfowl, less is known about its effects on the over-water nesting guild (i.e. diving ducks), which there exists no current management specifically for over-water nesting ducks. From 2015-2017 in the Prairie-Parkland Region of Manitoba, we assessed daily-survival rates of over-water duck nests in areas where efforts to reduce the local predator community were being coordinated and compared them to nearby areas where no targeted predator management occurred. Given the challenges in locating over-water nests, we also investigated an alternative method to estimate productivity using multiple rounds of surveys to derive brood-pair ratios. Brood-pair ratios have been widely used to index productivity, but biases associated with detection probabilities (the probability a pair or brood is seen during a survey) can result in underestimating abundances, especially for broods. We conducted replicate surveys to estimate detection probabilities of broods hatched from over-water nests to include in brood-pair ratios and compared productivity estimates derived from adjusted brood-pair ratios with estimates calculated from nest success on the same sites. We located and monitored 1,673 over-water nests from a variety of duck species to derive daily-survival rates and nest success estimates using Shaffer’s logistic-exposure methods and included a variety of covariates hypothesized to influence the probability a nest was successful. Nest success ranged from 14-48% across trapped and control sites, yet no overall trapping effect was observed despite numerous predators being removed from the landscape. Temporal effects such as nest-age and initiation date were influential predictors of daily-survival rates, which increased with nest-age and as the nesting season progressed. Detection probabilities for broods were estimated from 1,915 unique encounter histories using Huggin’s closed-capture methodology, which also incorporated covariates hypothesized to influence detectability. Detection probabilities were >50% for broods during all survey rounds and most heavily influenced by the percentage of the inundated wetland unobstructed for viewing broods. Sightability-adjusted brood-pair ratios for single-species were weakly correlated with nest success, however, combining all diving-duck species resulted in strong correlations between sightability-adjusted brood-pair ratios and nest success on each site-year (n = 18, R2 = 50%, P = 0.0005). Therefore, sightability-adjusted brood-pair ratios when multiple species are combined provide a useful alternative to index local diving-duck productivity when estimating nest success is unfeasible.