YOUNG, R.L.*; BADYAEV, A.V.; University of Arizona; University of Arizona: Mechanisms behind maternal favoritism in a passerine bird: Rate and duration of yolk development in relation to embryo sex and laying order

All birds lay at most one egg per day until their clutches are completed. However, because the formation of a single egg often takes a long time, the development of some eggs in the ovary overlap substantially. Because egg yolk maturation (rapid yolk deposition) requires large amounts of nutrients to be transferred from mother�s diet and/or body tissues, simultaneous maturation of several developing follicles in the ovary might limit the allocation of resources into each one. However, yolk size has a profound effect on embryo development, as well as hatchling size, growth, and survival. As a result, mothers routinely allocate different amounts of nutrients into the eggs of different laying order and sex. Yet the proximate mechanisms for such biased allocation are not known. Here we examine how breeding female house finches (Carpodacus mexicanus) solve the trade-off in resource allocation among competing eggs in the population where strongly sex-biased egg laying order is favored by natural selection. We measured duration of yolk development by staining the yolks of freshly laid eggs. Staining exposes layers in the yolk, each corresponding to 12 hours of lipid deposition. We examined the duration and intensity of lipid deposition in relation to the sex of the embryo, to the laying order, and to the extent of overlap with other maturing eggs. Furthermore, we investigated variation in this allocation due to female nesting phenology, incubation strategies, and associated hormonal profiles. Our results provide important insights into the proximate mechanisms that underlie the evolution of maternal strategies in birds.