Landscape risk predicts depletion of flooded unharvested corn by waterfowl

Animal foraging strategies are formulated in a complex decision-making matrix that is predicated on balancing energy intake and expenditure within a landscape of predation risk. Game species encounter dynamic risks during hunting season as ephemeral predators (i.e., hunters) alter the predation risk landscape daily and seasonally. Predators and prey often concentrate around food sources during hunting season, further complicating the balance of energy intake and risk minimization for prey species. For example, many wetland managers provide energy-rich food resources, such as unharvested grain, that attract waterfowl. Hunter access to these areas varies and alters the inherent risk that wintering waterfowl face to use these resources. We hypothesized use and subsequent depletion of unharvested flooded corn fields would be influenced by proximate anthropogenic disturbance and predation risk, shifting the bioenergetic landscape for waterfowl. We first sampled 145 unharvested flooded cornfields under different hunting access regimes to estimate corn biomass in October 2019 and 2020 in western Tennessee, USA. We then returned to 30 of those fields biweekly during fall-winter 2019–2021 to estimate depletion rates in 60 fields. We modeled depletion rate as a function of hunting risk and forage accessibility by including the variables of corn ear height above water surface, field size, field ownership type, and field distance from sanctuary in our statistical model. Biomass estimates derived from initial surveys indicated private fields that were hunted provided the greatest corn biomass (7,134 ± 448 kg/ha), followed by public fields that were hunted (5,272 ± 320 kg/ha) and finally sanctuaries closed to hunting (3,995 ± 371 kg/ha). Corn biomass was depleted 2–4 times faster on sanctuaries than on hunted fields during November and December but 2 times faster in hunted fields than in sanctuary fields in January. Depletion rates increased as surface water came closer to corn ears but were unaffected by field size or field distance from unhunted sanctuary. Sanctuary fields were devoid of corn by the end of January, whereas 55% of public and 50% of private hunted fields still had corn remaining on 15 March, by which time most ducks had likely initiated migration. Private lands sampled in 2020 contributed nearly 7 times more energy than assumed in the 2015 Lower Mississippi Alluvial Valley Joint Venture energetic allocation models. Despite being devoid of hunters at night, hunted lands are not perceived by foraging waterfowl to be nocturnally similar to sanctuary areas during hunting season. In turn, hunted fields were exploited by ducks later in the season when sanctuary resources were scarce, providing available biomass for migrating birds. It may be important for managers to retain water on hunted fields through early spring given the lack of energy-dense foraging resources available on sanctuary areas. Conservation planners should consider resource depletion as a multiplicative function of hunting risk and abundance when allocating foraging habitat objectives for waterfowl in the non-breeding season.