Yolk steroids and the development of the hatching muscle in nestling European Starlings

Hatching asynchrony in avian species often leads to the establishment of a size hierarchy among nestlings. Because the youngest nestlings in a brood are often at a significant disadvantage with respect to competition for parentally-delivered food, one might expect to find that mechanisms have evolved to alleviate the detrimental effects of hatching asynchrony and enhance the survival of last-hatched offspring. The musculus complexus, or hatching muscle, is responsible for breaking the shell during hatching and for dorsal flexion and extension of the neck during begging. An increase in its strength in last-hatched nestlings could therefore reduce the duration of hatching or enhance the ability to beg for parentally-delivered food, thereby enhancing the competitive abilities of those nestlings. I previously found in the Red-winged Blackbird Agelaius phoeniceus that the mass of the complexus increases with laying order and that its development is mediated by the actions of yolk testosterone. I investigated the hypothesis that yolk steroids also affect the development of the complexus in the European Starling Sturnus vulgaris. I asked, first, whether the mass of the complexus increases with laying order and, if so, whether this increase is at least partially attributable to a corresponding increase in the concentration of yolk testosterone. Because exogenous progesterone increases the water content of the complexus in the domestic fowl Gallus gallus I also asked whether an increase in complexus mass, if present, could be partially attributable to an increase in progesterone concentration across the laying order. I found that both yolk testosterone concentration and the mass of the complexus increased with laying order; however, there was no variation in the concentration of either yolk progesterone or the proportion of water in the complexus across the laying order, There were no differences in the concentrations of either androstenedione or 5 alpha -dihydrotestosterone across the laying order.