Detection of forest grouse by mammalian predators: A possible explanation for high brood losses in fragmented landscapes

Abstract

We used hunting dogs and man to simulate the searching for nests and broods of forest grouse, i.e. capercaillie Tetrao urogallus and black grouse Tetrao tetrix, by mammalian predators. Our aim was to find out if and how forest fragmentation affects the searching efficiency of predators. In total, we found 73 capercaillie and 35 black grouse nests and 20 young capercaillie broods. We calculated that a mammalian predator will detect a capercaillie nest if closer than 1.6 m (95% C.I.: 0.7 - 2.2), a black grouse nest if closer than 1.1 m (95% C.I.: 0.8 - 1.6), and a capercaillie brood if closer than 39 m (95% C.I.: 17 - 89). Nests were distributed in all habitat types, whereas broods were restricted to specific brood habitats. Due to this and the difference in the detection radius between nests and broods, we estimated that the predator gain of searching for broods in brood habitat is about 80 times higher than the gain of searching for nests which are situated in all habitat types in our study area. As young broods concentrate in highly restricted habitats, the predator gain of searching for broods increases exponentially with the loss of brood habitat, whereas it decreases linearly with increasing nest predation. We discuss this mechanism as a possible process explaining the observed decline in capercaillie populations in fragmented forests and consider its implications for grouse management.