If you have been useing your DSLR (Digital Single Lens reflex) camera in auto mode then here is a lesson for you.
First an explanation about aperture and how it affects your final photograph.
Inside your lens there is a diaphragm that controls the amount of light reaching the image sensor. The diaphragm functions much like the pupil of the eye – it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.
Higher diameters (lower f numbers) alow more light to pass through but have a shallow depth of field, while smaller diameters (higher f numbers) allow less light through but more detail farther away from the point of focus will be sharp.
Your camera measures aperture in F Stops ( F/N.N ). The n.n represents the F Stop Number. The lower the numbers are the the wider the diameter and higher numbers mean a tighter diameter.
You are taking a photo of a bird and there are some grass in front of the it that you want to blur out a little but still keep the bird sharp. What you want to do is use the lowest aperture possible that will keep the bird in the best focus.
For proper exposure of the image a precise amount of light striking the recording media is required . At lower F Stops this light will accumulate faster than higher ones. Here is where the shutter speed comes in.
In aperture priority mode the camera will you chose how long to keep the shutter open. You
can usually adjust the exposure either with a wheel (optimal) or with a set of up and down buttons.
Try focusing you camera on the bird and taking a shot. Keep adjusting until you get the result you are looking for. Yes maybe until it flies away, but keep practicing you will get the hang of it.