The different types of structures built by males of different populations of bowerbirds are likely the result of

The different types of structures built by males of different populations of bowerbirds are likely the result of; differences in female choice of preferred male courtship displays.

Bowerbirds are birds that demonstrate special courtship behavior. When the males want to attract a mate, they dance to the female or they construct a structure (known as bowers) and beautify it with sticks and attractive colored objects. The quality and colors of decorations the males use on their bowers attract the females, and this way, the females select the male that has their preferred bower for courtship. The male also rearrange his bower based on success and response of the female.

the denominator has a very specific role.