Many bird photographers make common mistakes in their compositions which can sometimes be avoided and may result in a better photo.  Below is what I consider to be some of these mistakes:

Cropping or not including enough space for a birds unseen legs - especially water shots.
Incorrect - Spoonbill pictured has longer legs than image crop allows for	Correct - The bird's hidden legs do not extend past the bottom of the image

Taking a picture of a bird that is in both sun and shade which can cause exposure problems.
Incorrect - Top of bird is in sun and front of bird is in shade causing blown highlights (ie loss of detail in whites)	Correct - Bird is evenly lit in morning sun making highlights controllable.

Getting a shot where the background is too 'busy' and the bird is not prominent.
Incorrect - Background is so 'busy' it draws attention away from main subject	Correct - Bird situated against a clean background using low depth of field.

Not cropping the photo correctly to suit the composition.
Incorrect - When cropping or composing the photo it is usually preferred to leave additional space in the direction the bird is looking or flying.	Correct - Bird has more space in front to 'walk into'