In the past, I often used flash to augment daylight and create a more stylised look. However, there are times when introducing flash can be too stressful for the animal, so I must rely on natural light. For instance, when photographing the Rufus Hornbill for my Bird project, placing a flash above it would most likely have been too scary.
For my Dog book, I took an image of a Springer Spaniel with pheasants flying up from the bracken on a misty morning. To achieve the desired look, we combined flash with colour-balanced gels that matched the scene's ambient light.
When shooting in the studio, I often need to freeze movements, such as a bird in flight or an animal leaping, with a flash system that captures speeds up to 1/10000 a second. Although I sometimes use continuous light sources for stills, it is mostly used for capturing motion.