Evolutionary Ecology , 1995 , 9, 3V37 Contrasting phenogrpic correlations in food provision of male Tengmalm's owls (Aegolius funereus) in a temporally heterogy'ous environment /n" HARRI HAKKARAINEN* And ERKKI KORPIMAKI Laboratory of Ecological ZooloSy, Depaftment of Biology, University of Tu*u, FIN-205U Turku, Fmldnd Surnmary We examined the foodprovision rateof male Tengmalm's owls, Aegolius funererc, during one3 year vole cycle consisting of consecutive low, increase andpeakf vole years. The datawerecollected in the mid- nestling period when males provisioned the whole family. In the low voleyear, males with a low loading index (g/cm2) of flying area fedtheiroffspring more often than did males with a high loading index, whereas in the p€ak voleyearthe opposite trendwas evident. Similar relationships werefoundin the food mass provisioned to thenest. In theincrease vole year, male body size had no effect on feeding efficiency. In the peak vole year,when large voles areabundant, healy males preyed on larger voles thanwere generally available in theirterritories, indicating thatlargeness may increase strike power in hunting attempts. In the lowvole year. when breeding iscostly due to foodscarcity and extensive hunting area, small males aremore economical fliers and efficient hunters than large males. Thecontrasting trends in correlations between male size and feeding efficiency in years of voleabundance versus scarcity suggest that no fixed phenotype may most efficiently cope with variable foodsupply. Keywords'. owl; 1-4 year volecycle; py'rental efforti reversed-size dimorphism Introduction Life-history theory traditionally has concentratedon predictions about optimal phenotypes adapted to a stabl€environment.Recently,a theory of evolution in heterogenous environments has stressed the significanceof environmental modification of the phenot)?e, in which phenotypic optima obviouslydiffer across a large rangeof environmental variations (for reviews, see Bradshaw, 1965; Stearns, 1992). If selection pressure varies with time in a given environment somephenotypictraits may be of contrasting value as conditionschange, that is, a trait may be beneficial under someconditions, but detdmentalin other situations. This may particularlyapply to specifies subsisting on food sources with high multiannualfluctuations in food abundance (for suchfood sources, see,e.g. Kalela (1962),Keith (1963),Finerty (1980),Angelstam et al. (7985) and Neuvonen (1988). Tengmalm's owl (A egolius funereus),is a small nocturnalavianpredatorwhich feeds mainly on voles of the ge \s Microtus and Clethrionomys (e.g.. Korpimiiki, 1981, 1988a). In Eurasian coniferous forests, the densitiesof these voles oscillate in 3-5 year population cycles (e.g. Hansson andHenttonen, 1985;Hanski etal., 1993). In western Finland, small male Tengmalm's owls perform bestin low vole years whereas large malesare the most successful breeders in good years. In years of intemediate food abundance, light and heavy malessucceeded equally well (Hakkarainen and Korpimiiki, 1991),as measured by the number of nestlings produced,which ' To whom corresDondence shouldbe addressed. lQ, l" 0269-7653 @ 1995 Chapman& Hall