Contrary to much, if not all, of the advice you have heard or read, when it comes to composing pictures of wildlife you do not always need to fill the frame with your subject(s).
In fact leaving plenty of room around the subject enables you to achieve a number of things within a photograph, including, showing the subject in the context of its natural environment, creating a sense of scale, and providing space within the frame area for other material, such as over-laid text, which can be an important consideration if the picture is intended for publication.
If you want to show the subject’s environment you will need to consider how the depth of field will affect the way other elements in the frame are rendered, so think about using a smaller aperture (higher f/# number) to increase the depth of field. Likewise, if you wish to add scale to a shot you will need to include a recognizable object in the frame, for example a feature in the landscape such as a tree, in addition to the main subject, so consider using a shorter focal length, and position the subject low in the frame, as this will enhance the sense of scale.
Little Bee-eater, South Africa
Technical details: Nikon D300s, Nikkor AF-S 200-400mm f/4 (400mm), 1/640 f/5.6 ISO200