Variation in productivity and territory occupancy in an Eagle Owl Bubo bubo population

Abstract

Source-sink dynamics refer to systems with some units as sources of juveniles, and others as sinks. The same pattern can theoretically occur at a small spatial scale, within single populations. Under these circumstances, varying quality of breeding habitats/territories determines different levels of survival and/or reproduction, which in turn result in varying contributions of territories to the population dynamics. Territory occupancy has been proposed as an indirect measure of habitat quality. Here, we used territory occupancy and productivity from a long-termmonitoring of an Eagle Owl Bubo bubo population in NW Italy to (i) show how different contributions to population dynamicsmay characterize territories over limited geographical scale in a strongly territorial species, and (ii) investigate the potential use of territory occupancy as a proxy for habitat quality in this species. Three out of 10 territories appeared to producemost fledglings (potential sources), whereas the others were characterized by extremely low productivity (possible sinks). The overall productivity is likely to nearly balancemortality, but the general equilibriummasks complex variation in the contributions of territories to the population dynamic. Territory occupancywas affected by average productivity, and thus long-term occupancy could be used as a proxy for habitat quality. In long-lived and scarce territorial species, such as the Eagle Owl, preservation of suitable conditions at key territories can be crucial for the population survival.