Seasonal survival rates and causes of mortality of Little Owls in Denmark

Survival rate is an essential component of population dynamics; therefore, identification of variation in mortality rates and the factors that influence them might be of key importance in understanding why populations increase or decrease. In Denmark, the Little Owl Athene noctua, a species strongly associated with anthropogenically modified landscapes, is declining fast and may soon face extinction. The population decline is ultimately associated with reduced survival of independent offspring, but reduced survival rates of adults may possibly contribute to the observed decline. To explore the causes of current survival rates, we estimated age- and season-specific survival rates and causes of mortality in Danish Little Owls on the basis of ringed birds 1920-2002, radio tagged adult and juveniles 2005-2008 and nest surveys 2006-2008. We estimate that 32 % of all eggs fledge and survive to 2 weeks post hatching (age of ringing) and 47 % of the nestlings from ringing to fledging. Fifty-five percentage of the radio-tracked fledged young survived to dispersal, i.e. a total survival rate from egg to dispersal of 8 %. Analyses of combined ringing and radio tracking data showed a lower survival rate in the 1st year of life and a much lower rate in the first 3 months of life. Furthermore, the analyses indicated that survival was lower in the winter months for ringing data during 1920-2002 but not for radio-tagged owls during 2005-2008 that experienced the highest mortality rates during the breeding season. In radio-tagged adults and fledged juveniles, accidents in buildings and other human infrastructures were responsible for two-thirds of all fatalities. Anthropogenic habitats currently comprise the nesting and roosting habitats for the last Danish Little Owls. The accidental deaths associated with these might to some extent be considered as a contributing factor to the present negative population growth rate of this population.