Predation on artificial ground nests in a subarctic ecosystem

Abstract

Studies of gallinaceous bird species stress the importance of high nest losses as a factor influencing life histories and populations. Accordingly knowledge of the mechanisms affecting nest losses is essential for understanding the population dynamics. The aim of this one year study was to estimate predation rates on artificial ground nests in ptarmigan habitats, along a landscape gradient which spans from subarctic birch forest to the low-alpine zone. We particularly wanted to investigate the importance of red fox (Vulpes vulpes) as a nest predator, and the study area was divided in three sub-areas according to the red fox density recorded during a winter snow track survey. Artificial nests (track boards) were placed in three different habitats (birch, edge and low-alpine). Vertical vegetation structure along the gradient from the birch forest to the low-alpine zone was surveyed and analyzed to assess how the forest structure could affect the predation rates. Predation rates resulting from different species were analysed using logistic regression models.

Total predation rates over all habitats, locations and periods ranged from 47.4 % to 77.5 % and were remarkably constant over all variables. Predation by avian predators was consistently high over both trial periods (59.1 %) and mammalian predation was consistently low (5.6 %). The constant predation rate was mainly due to omnipresent corvids, especially the hooded crow (Corvus cornix), which dominated among the predators in all habitats and locations in the study. The high predation by avian relative to mammal predators may be due to differences in landscape use and searching effort. Differences between locations were insignificant for all species except for raven (Corvus corax) that showed significantly higher predation between two of the locations. Red fox played a minor role as nest predator and no relation between nest predation rates and red fox density was documented. From the perspective of nest predation risk on ground nesting birds like willow ptarmigan (Lagopus lagopus), they would not be expected to be selective with respect to nesting habitats.