A population of male spotted bowerbirds that uses fruits in their sexual displays have developed a curious relationship with the plants. The birds indirectly cultivate the plants — the first example of cultivation of a non-food item by a species other than humans.
A team of scientists from the Universities of Exeter (UK), Potsdam (Germany), Deakin and Queensland (Australia) studied bowerbirds at Taunton National Park in Central Queensland. The research was published in April in Current Biology.
Several species of bowerbird are known for the elaborate and ornate bowers built by males to attract females and entice them into mating. Male birds often gather brightly-colored objects to decorate their bowers and arrange them in particular ways.
The researchers found more Solanum ellipticum, or potato bush, plants around bowers than in other locations. It turns out that male bowerbirds are not selecting bower locations with a high number of plants, but rather more of the plants grow around bowers due to the activity of the birds.
Solanum plants have bright purple flowers and green fruit. Male bowerbirds collect the fruits to display in their bowers. Once the fruits shrivel and turn brown, the birds discard them nearby. The result is new Solanum plants popping up around the bower.
There is no direct evidence to suggest that male bowerbirds are intentionally cultivating Solanum plants around their bowers. Instead, it seems that the growth of the plants is an indirect result of the birds' fastidiousness. Part of a male bowerbird's routine for keeping his bower clean and attractive is to clear the area around it of weeds and leaf litter, which creates an ideal habitat for the germinating Solanum seeds.
Male bowerbirds can maintain a bower in the same location for up to ten years, and may benefit from establishing a local supply of Solanum. The research team found males with especially fruity bowers had high mating success. These males placed Solanum fruit disproportionately in the center of their bowers and held the fruits in their bills during displays to females. Researchers also observed bowers with many fruit in one year tended to be surrounded by more Solanum plants the following year, perhaps granting those males a long-term fruit supply.
Interestingly, the scientists never observed the bowerbirds eating the fruit and found no Solanum seeds in the birds' droppings. Their use appears to be purely decorative.
The bowerbirds' aesthetic taste is even changing the appearance of the fruits. The fruits from plants close to the bowers were brighter green than those found on other plants. When the researchers tested the male birds, they preferred the greener fruits.