Light is all important in photography. Without it, no picture. When photographing wildlife, you can use natural or artificial light to illuminate the colors of the bird. Feathers are basically little prisms that reflect light. In the case of this Ruby-throated hummingbird, the light in coming in from the side, not directly head on and the gorget is illuminated red. Taking this picture. I am not quite directly head on, I am off to the left a bit around 45 degrees to the bird, allowing me to see the red. The angle of incidence = Angle of reflection.

In this second image, the hummingbird slightly moved his head in relationship to the light and the angle of reflection changed, not illuminating his gorget fully. I have not changed my position in relationship to the hummingbird at all.

Here is yet a third example of how the The angle of incidence = Angle of reflection. The hummingbird turned yet again only allowing certain feathers in the gorget to reflect the light back. This physics concept is why sometimes the area of this hummers throat will be seen as black and at others flashes red. It is all about the light.