Objects of desire
In courtship, gifts are always appreciated.
In Australia the satin bowerbird has an eye for the presents his partner adores. In this case her favourite colour is blue.
His bower is not a nest - it's a place of seduction and he must keep up its maintenance to impress any females that come to call. She visits all the bowers in the neighbourhood to judge each male on his home decorating skills. She's hard to impress.
An experienced male won't accept rejection lightly, he just works harder to win her over. The more blue trinkets he accumulates the more he turns her eye. His interest in home decorating is purely biological, the harder he works the more he proves his fitness. When he has amassed the bowerbird equivalent of wealth, he has proved his genetic worth.