Assessment of relative mortality rates for two rapidly declining farmland owls in the Czech Republic (Central Europe)

Abstract

Anthropogenic mortality has a considerable impact on populations of long-lived species, such as raptors, which increasingly inhabit human-dominated landscapes. Here, we analyzed long-term mortality data for two rapidly declining owls, Little Owl Athene noctua and Barn Owl Tyto alba, in the Czech Republic. We evaluated relative mortality rates with respect to owl age, month of carcass recovery, and two time periods (before and after year 2000). We examined 961 mortality records (199 Little Owls and 762 Barn Owls) derived from six distinct database sources totally spanning the period of years 1934–2017 and the entire Czech Republic. Natural causes, entrapment in vertical hollow objects and drowning in liquid reservoirs (entrapment), and collision with vehicles accounted for the highest proportion of mortality cases in Little Owl, while collision with vehicles and entrapment represented the most important mortality sources in Barn Owl. Relative mortality rates in Little Owl caused by entrapment, non-vehicle collision, electrocution at power lines and confinement in buildings increased after the year 2000. In turn, the relative mortality rate due to collision with vehicles increased after 2000 in Barn Owl. Persecution, collision with vehicles, and entrapment accounted for higher relative mortality rates in first-year than adult Little Owls. In Barn Owls, higher relative mortality rates due to collision with vehicles and entrapment were detected in adult compared to first-year birds. Finally, relative mortality rates differed between age classes according to the month of carcass recovery for both species. For Little Owl, the highest relative mortality rates in first-year individuals were detected during July and September, whereas adult Little Owls suffered the highest relative mortality rates during March, November and December. In Barn Owls, the relative mortality rates of first-year individuals peaked in November and December, whereas adult birds suffered the highest relative mortality rate during July, January and February. This study strongly suggests that reducing the risk of anthropogenic mortality may be crucial to halt the decline of Little Owl and Barn Owl populations.