Maternal effects on offspring Igs and egg size in relation to natural and experimentally improved food supply

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1. Maternal effects have been suggested to function as a mechanism for transgenerational plasticity, in which the environment experienced by the mother is translated into the phenotype of the offspring. In birds and other oviparous vertebrates where early development is within the egg, mothers may be able to improve the viability prospects of their offspring at hatching by priming eggs with immunological and nutritional components. 2. We studied how resource availability affects maternal investment in offspring by feeding Ural owl (Strix uralensis, Pall.) females prior to egg-laying in 3 years of dramatically different natural food conditions. 3. Supplementary feeding prior to laying increased body mass and the level of Igs of females measured at clutch completion. Supplementary fed Ural owl females laid larger eggs than control females, and had offspring with higher levels of Igs at hatching compared to offspring of control females. 4. We found variation in maternal allocation of resources to the eggs in response to environmental conditions: during a year of rapidly declining food abundance, maternal Igs in hatchlings were higher, whereas egg size was smaller compared to years with a more stable food supply. 5. Egg size had a positive effect on offspring body mass at fledging, whereas Igs at hatching did not affect Igs at fledging. 6. We conclude that maternal body condition and maternal Igs, as well as hatchling Igs and egg size are limited by food resources during egg production. Hatchlings rely on maternally derived Igs and, hence, our results suggest that mothers with high levels of Igs passively transfer higher Igs levels to their eggs instead of active manipulation of Igs levels in eggs. Ural owl egg size appears to be highly sensitive to short-term changes in food abundance, with important consequences for nestling growth.