Hatching asynchrony and maternal androgens in egg yolks of House Wrens

Synchronously and asynchronously hatching clutches of House Wrens Troglodyt es aedon usually do nor differ in reproductive success. Thus late-hatching nestlings in asynchronously hatching clutches somehow overcome any age- and size-related disadvantages of hatching after their nest-mates. One possibl e way for them to do this is for female House Wrens to add maternal androge ns to the yolk of late-hatching eggs. We tested this hypothesis in a wild p opulation of House Wrens that produces both asynchronously and synchronousl y hatching clutches. Yolks of eggs from both types of clutches were biopsie d and the eggs returned to their nests to hatch. Radioimmunoassays revealed that total androgen levels in the yolk varied within and among clutches. H owever, total androgen levels in yolks did not vary predictably with egg po sition in either synchronously or asynchronously hatching clutches. Thus. d eposition of androgens in yolk did nor follow the expected pattern based on the potential for sibling competition in House Wrens.