Photographers often have to work with less than ideal conditions. This was true for me the other morning at Morton's Wildlife Refuge. I planned to get there in time for sunrise. The place is 90 minutes away from my apartment but I hit some unexpected traffic. It quickly became apparent that I would be spending the magic hour behind the wheel of my Nissan. Knowing the light would be less than ideal, I considered just turning around and going home. I decided to stick it out and make the best of it. The forecast called for clouds, and I hoped that maybe they would rescue me. Bright, cloudy days are perfect for bird photography. By the time I reached the park it was clear blue skies. Now, the challenge was how can I make decent bird images with harsh overhead sunlight? Well, outside of finding a bird in the shade, the only other thing you can do is to use fill flash. The trick is to underexpose your background by a stop or so, and use the flash in conjunction with the ambient light. The female cardinal was backlit by the sun, and the front of the bird was lit by a -1 pop of fill flash.
The male cardinal also benefited from a little flash. Notice the small catch light in the eye.
I find myself using my external flash more and more for nature scenes. It really saves the day when you are shooting in less than ideal conditions. I use the Canon 580EXII because it's powerful enough to use with a 400mm lens, and has a very quick recycle time which allows it to keep up with my camera.