A certain species of bird has two basic varieties crested and noncrested  The birds, which generally live in flocks that contain only crested or only noncrested birds, tend to select mates of the same variety as themselves  However, if a bird that is raising a flock in which all other members are crested is later moved to a mixed flock, then that bird -- whether crested or noncrested -- is likely to select a crested mate
This fact indicates that the birds' preference for crested or noncrested mates is learned rather than genetically determined
Birds of other species also tend to show preferences for mates that have one or another specific physical feature.