Abstinence makes for bluer feet

Source: nationalgeographic.com
When male blue-footed boobies do not reproduce for a year the blue colour of their feet becomes more intense. This increases the chance of finding a partner. The blue colour is attractive to females.
The blue feet of the bird are important in the mating ritual. The males display them to lure females. As the blue-footed boobies become older the blue colour becomes less intense. This is also seen in other animals whose colour is an important aspect of the mating ritual.
Researchers of a Spanish and Mexican university discovered that when a male does not mate for a year, its feet have a more intense colour the next year. Thus the male becomes more interesting to the females.
In blue-footed boobies the males are also actively involved in breeding the eggs and taking care of the young. The researchers think that when a male does not have to take care of any young for a year, it is better able to recover and has more energy to make its feet nice and blue.