Beyond maternal effects in birds: responses of the embryo to the environment.

Embryonic growth and development are impacted by environmental conditions. In avian systems, parents tightly control these environments through provisioning of nutrients to the egg and through incubation. Parents can influence embryonic development through egg size, eggshell conductance, hormones, or other substances deposited in eggs and through the onset and temperature of incubation. In addition to these parental influences, evidence suggests that avian embryos are able to perceive and actively respond to their environment during incubation and adjust their own development. Evolution of embryos' responses to developmental environments in birds can be understood in the context of parent-offspring conflicts. When parental investments favor future reproduction over current reproduction, current offspring pay fitness costs, which result in strong selection for offspring that can respond to developmental environments independent of their parents. Here, we review literature indicating that avian embryos actively respond to maternally derived components of the egg, vocalizations, and differences in day length, and we explore these responses in the context of three situations where the consequence of these environments to the fitnesses of offspring and parents differ: the degree of synchrony in hatching, the deposition of hormones in yolks, and seasonal timing of breeding. However, the adaptive significance of responses of embryos to developmental environments arising from parent-offspring conflict has not been adequately explored in birds.