How do you know birds are females or males?

Most hummingbirds exhibit sexual dimporphism meaning that the male looks different than the female. Most male hummingbirds are much more brightly colored than their mates. Females tend to be very drabbly colored so that they blend into their surroundings better. Blending in is particularly helpful when the female is sitting on a nest. If she were brightly colored like a male, she would draw attention to herself and risk putting herself and her chicks in danger.