Predators control post‐fledging mortality in tawny owls,Strix aluco

Abstract

Despite its recognition as an important source of variation in recruitment probability, the ecological processes leading to mortality between fledging and independence are poorly studied. Accordingly, the proximate and ultimate impact of bottom‐up (food limitation) and top‐down factors (predators, pathogens) for individual survival as well as population productivity is largely unknown in most terrestrial birds. Survival and behaviour of 131 radio‐tagged tawny owls (Strix aluco) during the post‐fledging dependency period were studied for each of three years with high food abundance and three years of poor food supply in Danish deciduous woods. To identify the effects of food limitation, 32 young received extra food 2–3 weeks prior to fledging, as opposed to 99 young that were fed by their parents only. Thirty‐six percent of the young from control broods died between fledging and independence, primarily due to predation from raptors and mammals (predominantly foxesVulpes vulpes). Predation by mammals occurred within the first few days after fledging, with young leaving the nest at an early age being at particularly high risk. Young that had received extra food as nestlings were also at higher risk of being preyed upon by mammals, possibly because they were easier to locate on their smell. Total mortality risk (control broods) increased with fledging date from 14% in April to >58% in June due to an increasing raptor predation risk. Individual attributes such as sex, condition, immunocompetence or prevalence of blood parasites did not predict total or cause‐specific mortality risk. Survival during the post‐fledging dependency period was therefore primarily a function of variation in predation pressures, particularly from raptors. Increasing raptor predation of late broods appears to be an important selective agent for early breeding in the tawny owl.