I have not seen them very often nor observed their behavior, though I imagine they're prone to the same competitiveness and bad temperament as many other hummingbirds.
The day after I was first able to photograph a Costa's, I saw another hanging out by my herb garden, where I'd never seen one before. Perhaps this is a bit of synchronicity.
Like the juvenile photographed yesterday (picture below), this Costa's did not have his mature plumage yet, but he was farther along.