Yolk steroid concentrations decline in Japanese Quail ( Coturnix japonica ) eggs throughout incubation

The development of avian embryos is dependent not only on their genetic background but also on both external conditions and the maternal resources deposited into eggs. Specifically, maternally derived steroids in egg yolks have been shown to influence morphological development in avian offspring, but the effect of most yolk steroid hormones and their metabolites on embryonic growth and development remain poorly explored. We tested relationships between eggshell maculation, surface temperature, hormone concentrations before and during incubation, and embryonic growth in Japanese Quail ( Coturnix japonica ) eggs developing while artificially incubated in duplicate incubators. We detected 10 steroid hormones in the yolks during development and concentrations of all hormones declined to Day 15 (∼2 days before hatch). Steroid declines at Day 9 of incubation were related to eggshell surface temperature, with warmer eggs having higher concentrations of androgens and progestogens, and incubator assignment, as eggs were warmer on average in one incubator than the other. However, eggshell surface temperature was not related to eggshell maculation, embryonic size, or embryonic heart rate during development. These results provide evidence of yolk steroid concentration declines through incubation and suggest that environmental conditions (such as temperature or light) could alter embryonic hormone uptake and/or metabolism.