Breeding, survival, movements and foraging of tawny owls Strix aluco in a managed spruce forest : a spatial approach
In 1996-98,1 studied the spatial ecology of the tawny owl Strix aluco (L.) in Kielder, a managed spruce forest in Northumberland, northern England. I employed radio tracking in an individual-based approach, estimating densities of field voles Microtus agrestis, the owl's main prey in Kielder Forest, using a calibrated sign index. In both 1996, a low vole year, and 1997, an increasing vole year, juvenile mortality was high, and largely due to starvation and predation. 55.4% of the variation in the mean number of days survived post-fledging per brood was explained by a model comprising the variables mean clutch hatch date, brood size, and voles per hectare at the clear-cuts nearest natal nest boxes in the spring of breeding. Nine percent of non radio tagged juveniles were recruited in 1997-98, in comparison with none of the radio marked individuals. Post onset of dispersal, vole abundance explained 25.7% of the variation in the time that juveniles spent in different areas. Juveniles did not become more sedentary over time nor avoided roosting in occupied territories. Adult home ranges contained more grassy areas than expected from their abundance in the study area, but not from their abundance at watershed level. The absolute areas of grassy habitat and lengths of habitat edge that were included in home ranges varied widely. Range area was correlated with distance to the nearest clear-cut, but not with variation in estimated vole abundance. Nocturnal activity centred on clear-cuts, other grassy areas, and mature forestry plantations. Field voles constituted 59% of prey deliveries to two nest boxes. The time that owls spent at clear-cuts was not correlated with estimated vole abundance there. Tawny owls are generalist predators, but their spatial ecology was strongly influenced by the abundance of their main prey species and the distribution of habitats that supported it.