Manipulation of male attractiveness induces rapid changes in avian maternal yolk androgen deposition

Avian eggs contain maternal androgens that may adjust offspringdevelopment to environmental conditions. We review evidenceand functional explanations for the relationship between androgenconcentrations in avian eggs and male attractiveness. Experimentalstudies in captive birds show generally positive relationships,but results from correlational and experimental field studiesare less consistent, perhaps because they lack a within-femaledesign to control for confounding between-female variation.We analyzed the effect of male attractiveness on yolk levelsof maternal androgens in a wild bird, using a correlationaland experimental approach with a within-female design. We manipulatedthe sexually selected UV coloration of the crown feathers ofmale blue tits (Cyanistes caeruleus) after their female hadlaid the second egg and measured the subsequent effect on androgenconcentrations (testosterone and androstenedione) in the fifth,seventh, and ninth eggs relative to that in the second egg.Levels of testosterone, but not androstenedione, in eggs 5 and7 were higher for control (attractive) than for UV-reduced (unattractive)males. This effect disappeared in the ninth egg, coincidingwith the recovery of UV coloration after manipulation. Thissuggests that females are capable of rapid adjustments of testosteronedeposition in response to changes in their mate's ornamentalplumage. However, androgen concentrations in the second eggand pretreatment male crown coloration were not correlated.Possibly, the combination of relatively small variation in UVcoloration before treatment and the influence of unknown confoundingvariables in the correlative approach resulted in insufficientstatistical power to detect such a correlation.