Does feeding effort of Tengmalm's owls reflect offspring survival prospects in cyclic food conditions?

In an environment fluctuating in a predicatable manner with wide among-year variation in offspring mortality, fitness is largely influenced by the timing of reproductive investment. In vole-eating nocturnal Tengmalm's owls (Aegolius funereus), within-cycle variation in 1st-year survival of owlets is 3-fold as estimated by the recruitment probability of an offspring. It increases from the peak through the low to the increase phase of the vole cycle. We recorded prey delivery rates of males during a 3-year vole cycle using 4 h continuous recording at night. Males brought significantly more prey items per offspring in a low-vole year than in the increase and peak vole years. In the early night (10 p.m.-12 p.m.), males fed their families equally in the increase and peak vole years, whereas in the late night (0.01-2.00 a.m.) males reduced their feeding rate in the peak year but not in other years. Both prey number and prey mass per offspring were larger in the low and increase vole years than in the peak year, though in the peak phase food is most abundant. We suggest that feeding effort of site-tenacious, long-lived (mean lifespan 3.5 years) male owls culminates in the increase rather than in the peak phase of the vole cycle, because offspring survive better in the former phase.