Your cameras Aperture (measured in f-stops) which controls Depth of Field is one of the most important creative controls for your image composition. The human brain is designed to crunch information and by default directs the eye to the sharp (in-focus) parts of any image where information can be easily processed. In contrast out-of focus and blurred parts of an image are ignored.
Large Depth of Field
A large depth of field means that the image will be in sharp focus all the way to the background (image 1). This image compels the viewer to look around from foreground to background and absorb all the information.
Shallow Depth of Field
A small depth of field means that the image will be in sharp focus only where you direct it (image 2). In this case the focus is in the immediate foreground making the sparrow the subject of the image. This composition drives the viewer directly to the subject and has a more relaxing feeling to it since your brain is required to process only a small amount of information.