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

The nest defence behaviour of vole-eating Tengmalm's owls, Aegolius funereus, in the presence of a live American mink, Mustela vison, was studied. The study covered a 3-year vole cycle, where low, increasing and peak vole years followed each other in a predictable manner. First-year survival prospects of owlets in the increasing vole years were twice as high as in the low and peak years. Males may have adapted to local fluctuations in food abundance because of restricted natal and breeding dispersal, but nomadic females with wide natal dispersal may not possess such a response. As expected, male, but not female, nest defence was related to the survival prospects of offspring. Age-related differences in the intensity of nest defence were also studied. Surprisingly, a negative relationship was found between male age and the intensity of nest defence in the peak vole year. This suggests that before a vole crash, old males may have higher residual reproductive value than young ones or that old males may reduce their investment at a time when large investment in offspring is not beneficial.