Boobies blue feet get brighter the longer it has been since they last had sex. Blue feet attract females. http://tw2t.com/TO

In a study of the blue-footed booby, which is native to the Galapagos Islands and Ecuador, they found that when birds had a year off from reproducing, their feet became brighter in colour.

For female boobies, the more luminous the foot, the more desirable the male.