Differential maternal testosterone allocation among siblings benefits both mother and offspring in the zebra finch Taeniopygia guttata.

Parents are selected to preferentially invest in the offspring with highest reproductive value. One mechanism for achieving this is the modification of competitive asymmetries between siblings by maternal hormones. In many organisms, offspring value varies according to birth position in the brood, which determines survival chances and competitive advantage over access to resources. In birds, variation in yolk androgen allocation over the laying sequence is thought to modulate dominance of senior chicks over junior brood mates. We tested this hypothesis in zebra finches, which show a naturally decreasing pattern of within-clutch testosterone allocation. We abolished these within-clutch differences by experimentally elevating yolk testosterone levels in eggs 2-6 to the level of egg 1, and we assessed fitness measures for junior offspring (eggs 2-6), senior offspring (egg 1), and their mothers. Testosterone-injected eggs hatched later than control eggs. Junior, but not senior, chicks in testosterone-treated broods attained poorer phenotypic quality compared to control broods, which was not compensated for by positive effects on seniors. Mothers were generally unaffected by clutch treatment. Thus, naturally decreasing within-clutch yolk testosterone allocation appears to benefit all family members and does not generally enhance brood reduction by favoring senior chicks, in contrast to the widely held assumption.