Using fill flash properly can help lift your images up to next level. It brightens up the shadows for a more appealing image. In wildlife and bird photography with telephoto lens a flash extender will be needed. I have a Better Beamer flash extender I've had for years. Flash extender usually have a fresnel lens that helps your flash go longer distances. I have found the best setting is to set the camera to manual mode and set the exposure for the light conditions and the flash to 50 mm(some have success with TTL others with 24mm, experiment!!) and make sure your flash has high-speed sync on with a FEC starting at -2/3 and work in 1/3 increments down until you see the desired results. I have examples below with and without fill flash. Happy shooting!!
Without Fill Flash
With Fill Flash
Canon 40D
400mm 5.6
F/6.4
1/1250s
ISO 200