A long-standing interest concerns the function of conspicuous sexual ornamentation, using birds as models. Such ornaments are often important for male fitness, whereby more extremely ornamented males are more successful. We assume that some males are less ornamented because ornaments are costly, and level of ornament exaggeration is somehow limited by individual quality. We tested various proposed physiological and behavioural mechanisms and costs. We investigated how pleiotropic effects of hormones (testosterone in particular) and dependence of ornament expression on general condition or specific dietary components (antioxidants mainly) can act as such honesty-enforcing links. More recently, we showed that ornamented males might counter increased risk of predation by being more risk averse.

No fitness benefits of early molt in a fairy-wren: relaxed sexual selection under genetic monogamy?
Fan M, ML Hall, SA Kingma, LM Mandeltort, K Delhey & A Peters. 2017. No fitness benefits of early molt in a fairy-wren: relaxed sexual selection under genetic monogamy? Behav Ecol.

Early moult of the seasonal male plumage is an important sexually selected signal in other fairy-wrens. In purple-crowned fairy-wrens we could not identify any fitness benefits of producing the stunning male plumage early in the year, despite looking really hard, and a large sample size.

Seasonal male plumage as a multicomponent sexual signal: insights and opportunities.
Peters A, SA Kingma & K Delhey. 2013. Seasonal male plumage as a multicomponent sexual signal: insights and opportunities. Emu 113 (3), 232-247

This is a review of the various signalling modalities afforded by a seasonal, rather than year-round, conspicuous male plumage, as is commonly found in Australian fairy-wrens

No evidence for general condition-dependence of structural plumage colour in blue tits: an experiment.
Peters, A, RHJM Kurvers, ML Roberts & K Delhey. 2010. No evidence for general condition-dependence of structural plumage colour in blue tits: an experiment. J. Evol. Biol. 24(5): 976-987

We demonstrated lack of condition-dependence of a well-studied sexually-selected ornament, against expectations and theoretical predictions.

Condition-dependence of multiple carotenoid-based plumage traits: an experimental study.
Peters A, K Delhey, S Andersson, H van Noordwijk & MI Förschler. 2008. Condition-dependence of multiple carotenoid-based plumage traits: an experimental study. Funct Ecol 22(5):831-839

First experimental support for the predicted heightened condition-dependence of sexual ornaments using carotenoid-based bird plumage as a model.

Carotenoid-based bill colour as an indicator of immunocompetence and sperm performance in male mallards.
Peters A, AG Denk, K Delhey & B Kempenaers. 2004. Carotenoid-based bill colour as an indicator of immunocompetence and sperm performance in male mallards. J Evol Biol 17(5): 1111-1120

The yellow beak of the male mallard indicates not only the immune responsiveness of its bearer, but also the quality of his sperm, explaining why females might find yellow bills attractive.

Testosterone is involved in acquisition and maintenance of sexually-selected male plumage in superb fairy-wrens.
Peters A, LB Astheimer, CRJ Boland & A Cockburn. 2000. Testosterone is involved in acquisition and maintenance of sexually-selected male plumage in superb fairy-wrens, Malurus cyaneus. Behav Ecol Sociobiol 47(6): 438-445

A demonstration that testosterone is causally responsible for the production of a strongly sexually selected plumage combining careful observations and experiments.