Ecol. Lett. doi:10.1111/j.1461-0248.2009.01419.x (2009)

Sexual ornaments on birds, such as brightly coloured plumages, are thought to signal sperm quality, but how the two are linked has not been clear. Work by Fabrice Helfenstein at the University of Bern and his team confirms that more-colourful males are better able to protect their sperm from oxidative stress.

The authors followed a breeding population of great tits (Parus major). They ramped up the birds' workload, and thus their oxidative stress, by adding two nestlings to each brood.

Sperm from less-colourful males (pictured above, right) exhibited a greater drop in motility and more lipid damage than did sperm from their more-colourful counterparts (left). In addition, the sperm quality of less-colourful males improved when their diet was supplemented with antioxidant carotenoids.