1. Sweet vs. Sour Cherry Trees
Most sweet cherry trees require cross-pollination—pollen from a different cultivar of cherry tree. But sour cherry trees can self-pollinate, and are able to pollinate even if only pollen from cherry trees of the same cultivar or same trees are available—though pollen from different cultivars would boost yield. However, Japan's popular Somei Yoshino trees are sterile hybrids, and don't produce fruit. Since they don't produce seeds, they're grown by grafting in Japan.
2. Busy Bees Act as Pollinators
The most important pollinator for cherry trees is by far the honeybee. Honeybees are incentivized to visit the blooming flowers as they contain nectar, which is an important source of carbohydrates for the honeybees. Pollen sticks to the bees' bodies when they inject their tongues into the flowers' nectar. As they move from flower to flower with all the pollen, they effectively pollinate flowers as they come into contact with ripe stigmas.
The above video shows the typical pollination process—just in case we've forgotten our high school biology classes!
3. What Role Do Birds Play?
As for birds? All they have to do is pick the berries and eat them! Even the sour cherries can be an important source of food for small birds and mammals. Although the berries don't have much flesh, they're juicy and provide nutrients. Unfortunately for us, the sour cherries are a bit too sour, and our taste buds don't fancy them too much!
Interestingly, sparrows have adopted the role of nectar pirate since about half a century ago, and loot nectar without providing pollination services by cutting the flowers near the nectary. While it may not matter much if they're targeting Somei Yoshino trees since they're sterile, it must suck for the other cherry trees!