Right from the nest, young birds compete with each other for the food that the mother brings. They develop that competitive behavior right out of the egg. It's about survival. If you find a food source, protect it.
Watching and hearing their squeaky fights can appear like the most adorable thing, but in reality they will fight fiercely for what they like, sometimes resulting in serious injury.
This young male Ruby-throat hummingbird initially protected the Nasturtium flowers that were laid out below him, but soon discovered the feeder hanging from the little shepherd's hook he's sitting on. He was proud of his find, and discovered that he didn't have to work that hard to get an enormous amount of food from it. This became his guard perch. Any hummingbird that passed by or even thought about testing out his little territory got a squeak or two and a beak wag.
N.E. of Edmonton, Alberta, Canada. August 2017