It has the most controlling effect on them. It's what they think about from morning til night. They will seek it out no matter how difficult the circumstances, and they will fight for it with their life. The only thing it can't control is when they migrate.
Have you ever wondered why hummingbirds feed from flowers? It's the same with insects. Flowers were given the sweetest tasting nectar while hummingbirds were given the most overwhelming and overpowering desire for it. If their desire fell short of that need for flowers, the system of pollination would simply fall apart. We can eat something repeatedly for a few days and never want to touch it again, but through the lifetime of a hummingbird the desire for nectar just gets stronger. Every minute of every day is consumed with flowers and feeders. That little brain is wired and capable of remembering every flower and food source over the years. The proof of this is when hummingbirds show up to a location that held a feeder several months ago. How that tiny little brain is capable of remembering thousands of coordinates is simply beyond imagination. Their survival depends greatly on memory and obedience and that's exactly how nature was built. It does what it's supposed to do, when it's supposed to do it and for the reasons demanded of it. Without this kind of obedience the animal kingdom would crumble.
Here is a young male Ruby-throated hummingbird feeding from a Vining Nasturtium flower. N.E. of Edmonton, Alberta, Canada. August 2017