How are hummingbirds important to flowers?

Hummingbirds fly from flower to flower, sticking their beaks into the center to take sips of nectar. While sipping nectar, hummingbirds are inadvertently transporting pollen grains from previously sipped flowers. The process results in the cross pollination of different flowers, through the help of the hummingbird.

Basically: Hummingbirds help flowers pollinate each other, which allows flowers to reproduce.