Background is a key element in bird photography. One approach is to ensure the background colour complements the subject, but at the other extreme it can be just as valid to provide a striking contrast. Or you could try lighting your subject so that the background is either ½ stop under-exposed or over-exposed when compared to the perfectly exposed subject. Experiment with depth of field and try to avoid cluttered and complicated backgrounds. The standard approach to bird portraiture would be to use just the right aperture to ensure that every important feature of the bird in question is in focus. But how about using a minimal depth of field with just the bill and eyes in focus and everything else blurred?
To place this Sinai Rosefinch in environmental context a 105mm macro lens was used, to allow for a large depth of field
To throw the background out of focus this Sinai Rosefinch image was taken using a 600mm lens and 1.4x converter