Environmental correlates of annual survival differ between two ecologically similar and congeneric owls

Understanding how survival is affected by the environment is essential to gain insight into population dynamics and the evolution of life-history traits as well as to identify environmental selection pressures. However, we still have little understanding of the relative effect of different environmental factors and their interactions on demographic traits and population dynamics. Here we used two long-term, individual-based datasets on Tawny Owl Strix aluco (1981-2010) and Ural Owl S.uralensis (1986-2010) to undertake capture-mark-recapture analysis of annual survival of adult females in response to three biologically meaningful environmental variables and their two-way interactions. Despite the similar ecology of these two species, their survival was associated with different and uncorrelated environmental drivers. The main correlate of Tawny Owl survival was an inverse association with snow depth (winter severity). For Ural Owl, high food (vole) abundance improved survival during years with deep snow, but was less important during years with little snow. In addition, Ural Owl survival was strongly density-dependent, whereas Tawny Owl survival was not. Our findings advise caution in extrapolating demographic inferences from one species to another, even when they are very closely related and ecologically similar. Analyses including only one or few potential environmental drivers of a species' survival may lead to incomplete conclusions because survival may be affected by several factors and their interactions.