Do female Costa's hummingbirds really find this look attractive? Spring is the time to nest for the Costa's hummingbirds, before the desert gets too hot. Both males and females are looking for a partner, but it's up to him to impress her. Though his back shimmers with green, it‚Äôs not until we get her point of view that we see his true splendor. He flexes the iridescent feathers of his mantle until they become a glowing mask of violet.