Plants release nectar which attracts birds to come and eat. As the bird eats the plant pollen (plant sperm) will get on the bird, and travel with it to other plants were the pollen will falls off and impregnates another plants due to plants having both female, and male bodies thus causing cross-pollination. This happen thru natural selection because the plants has nectar and pollen due them wanting to be able to spread and have more offsprings.
