Egg yolk layers vary in the concentration of steroid hormones in two avian species.

Maternally derived steroid hormones are known to be present in the yolks of avian eggs; however, the physiological mechanisms involved in their deposition remain largely unexplored. Investigations of steroid production by avian follicles have demonstrated temporal differences in the concentrations of progesterone, 17beta-estradiol, and testosterone during yolk formation. Because yolk is deposited peripherally in concentric spheres as the oocyte develops, differences in the production of follicular hormones during yolk formation should be manifested in differences in the localization of steroids within layers of the yolk. To investigate this hypothesis we analyzed steroid hormone concentrations in layers of individual eggs of the dark-eyed junco (Junco hyemalis) and the red-winged blackbird (Agelaius phoeniceus). We found that in the dark-eyed junco the concentration of progesterone is significantly greater at the periphery of the yolk, while the concentration of 17beta-estradiol is significantly greater near the center of the yolk. We also found in both the dark-eyed junco and the red-winged blackbird that the concentration of testosterone remains constant from the interior to the intermediate layers of the yolk and then drops sharply between the intermediate and exterior layers. The patterns of hormone localization that we found agree with those predicted by studies of temporal changes in steroidogenesis in the maturing follicle of the chicken, thus suggesting that within-yolk variation in yolk steroid concentrations in the dark-eyed junco and the red-winged blackbird reflects temporal differences in the pattern of follicular steroidogenesis. Variation in the concentration of hormones among yolk layers presents a methodological concern for studies that involve the removal of yolk samples from viable eggs for subsequent hormonal analysis. This variation also has implications for the timing of embryonic exposure to steroid hormones.