ENVIRONMENTAL, PARENTAL AND ADAPTIVE VARIATION IN EGG SIZE OF TENGMALMS OWLS UNDER FLUCTUATING FOOD CONDITIONS

We studied egg size variation of Tengmalm's owls in western Finland du ring 1981-1990. The owls fed on voles whose population fluctuated in a predictable manner: low (1981, 1984, 1987, 1990), increase (1982, 198 5, 1988) and peak (1983, 1986, 1989) phases of the cycle occurred ever y third year. Eggs were largest in the increase phase of the vole cycl e, even though that voles were more abundant and egg-laying started ea rlier in the peak phase than in the increase phase. This suggests that owls invest mostly in egg size when vole abundance increases along wi th survival chances of offspring. Territory quality and female age had no effects on egg size, but egg size decreased with laying date in th e increase phase of the vole cycle. Egg size was significantly positiv ely related to the male age in the increase phase, but the opposite re lationship was significant in the peak phase of the vole cycle. The pa rtners of adult males also decreased their egg volume from the increas e to the peak phase, whereas the partners of yearling males produced t heir largest eggs in the peak phase of the vole cycle. This suggests t he importance of experience in prevailing food fluctuations. Possibly male Tengmalm's owls can adjust the intensity of courtship feeding not only in relation to the food abundance on their territories at the ti me of egg laying, but also to the survival prospects of their offsprin g. Phenotypic plasticity seems to play a substantial role, as the egg size repeatabilities of individual females and partners of individual males were low. Obviously, under cyclic food conditions, predictabilit y and in ter-generational trade-offs are important to life history tra its.