NEST DEFENSE OF TENGMALMS OWLS REFLECTS OFFSPRING SURVIVAL PROSPECTS UNDER FLUCTUATING FOOD CONDITIONS

The nest defence behaviour of vole-eating Tengmalm's owls, Aegolius fu nereus, in the presence of a live American mink, Mustela vison, was st udied. The study covered a 3-year vole cycle, where low, increasing an d peak vole years followed each other in a predictable manner. First-y ear survival prospects of owlets in the increasing vole years were twi ce as high as in the low and peak years. Males may have adapted to loc al fluctuations in food abundance because of restricted natal and bree ding dispersal, but nomadic females with wide natal dispersal may not possess such a response. As expected, male, but not female, nest defen ce was related to the survival prospects of offspring. Age-related dif ferences in the intensity of nest defence were also studied. Surprisin gly, a negative relationship was found between male age and the intens ity of nest defence in the peak vole year. This suggests that before a vole crash, old males may have higher residual reproductive value tha n young ones or that old males may reduce their investment at a time w hen large investment in offspring is not beneficial.