Maternal effects and urbanization: Variation of yolk androgens and immunoglobulin in city and forest blackbirds

Wildlife inhabiting urban environments exhibit drastic changes in morphology, physiology and behaviour. It has often been argued that these phenotypic responses could be the result of micro-evolutionary changes following the urbanization process. However, other mechanisms such as phenotypic plasticity, maternal effects and developmental plasticity could be involved as well. To address maternal effects as potential mechanisms, we compared maternal hormone and antibody concentrations in eggs between city and forest populations of European blackbirds (Turdus merula), a widely distributed species for which previous research demonstrated differences in behavioural and physiological traits. We measured egg and yolk mass, yolk concentrations of androgens (androstenedione (A4), testosterone (T), 5α-dihydrotestosterone (5α-DHT), and immunoglobulins (IgY) and related them to population, clutch size, laying order, embryo sex, and progress of breeding season. We show: i) earlier onset of laying in ...