Maternally derived yolk testosterone enhances the development of the hatching muscle in the red-winged blackbird Agelaius phoeniceus

Hatching asynchrony in avian species often leads to the formation of a size hierarchy that places last-hatched nestlings at a significant disadvantage . The hatching muscle (musculus complexus) is responsible for breaking the shell during hatching and for dorsal flexion of the neck during begging. An increase in its strength in last-hatched nestlings could mitigate the effe cts of hatching asynchrony by reducing the time required for hatching or en hancing the effectiveness of begging for parentally delivered food or both. We have previously found that yolk testosterone concentration increases wi th laying order in the red-winged blackbird Agelaius phoeniceus. In this st udy we investigated the hypothesis that yolk testosterone has anabolic effe cts on the development of the complexus, thereby influencing competition am ong asynchronously hatched nestlings. We found that both yolk testosterone concentration and relative complexus mass (complexus mass/nestling body mas s) increased with laying order and that these two variables were positively correlated in both newly hatched nestlings and in two-day-old broods. More over, direct injections of testosterone into egg yolks resulted in an incre ase in relative complexus mass, while injections of flutamide, a testostero ne antagonist, resulted in a decrease in relative complexus mass. Neither y olk testosterone concentration nor relative complexus mass differed between male and female nestlings.