The reliability of using counts of vocal begging young to estimate the number of surviving juvenile Tengmalm's Owls (Aegolius funereus) at the end of the post-fledging period

Abstract

Counting of calling males during territorial or nuptial displays is a method widely used for census of populations of wild vertebrates during the pre-breeding and breeding periods, particularly in relation to census of populations of wild birds. In principle, a similar method could be used for assessing numbers of surviving offspring, by monitoring of begging calls, but is used only rarely. We explored the possibility of assessing post-fledging survival of Tengmalm's Owl young by counting the number of individuals begging for food. This method did not however deliver satisfactory estimates of numbers of surviving fledglings and its accuracy depended on time of night, distance from the natal nestbox, and presence or absence of begging calls. We suggest that this method can be used in Tengmalm's Owls for rough estimates only and our finding could be applicable also for other, generally smaller, owl species whose fledglings exhibit quick dispersion from the nest site after the fledging.