The importance of functional responses among competing predators for avian nesting success

1. The relationship between the rate of predation and prey abundance is an important component of predator-prey dynamics. However, functional responses are less straightforward when multiple predators compete for shared prey. Interactions among competing predators can reduce or enhance effects of predation on prey populations. Because many avian populations experience high rates of nest predation, understanding the role of specific predators on nest mortality will lead to more informed conservation and management strategies which attempt to increase productivity by removing certain predators or managing habitat to limit their impact.
2. Our goal was to evaluate effects of specific predators and the influence of nest abundance on nest mortality.
3. We monitored snowy plover Charadrius nivosus nests across 7 years at two study areas in Utah, USA with remote cameras. We modeled predator-specific hazard rates for nest mortality in a Bayesian framework to assess relationships between competing predators and the role of nest abundance on predator-specific hazard rates.
4. We found that hazard rates for nest mortality by gulls Larus spp. decreased with increasing nest abundance, whereas nest mortality by foxes Vulpes spp. and ravens Corvus corax initially increased, indicating that dietary switching may occur when nests become more abundant. Nest mortalities of specific predators were often not independent and ranged between compensatory (e.g., mammalian mesopredators), and superadditive (e.g., avian predators) across the breeding season.
5. The non-independence between nest mortalities suggests that reductions in some predators may not translate to additive increases in overall nest success. Analyses of cause-specific mortality are rarely applied to avian nests, but examination of interacting impacts among competing predators on nest survival may provide insight into specific drivers of avian population dynamics.