Here comes the post processing softwares handy and the bird can be cropped to rectangle or vertical format, as it suits the placement of the bird more beautifully, not because u want to place it somewhere else u want. This comes under composition rules which depicts the pic aesthetically superb.
Many people do not like their sitting on electric wires, poles as they do not represent the natural view of the trees, jungle. But, in cities it is compelling, not to miss the shot, whether the perch may be artificial, as in the above picture.
METERING: I GENERALLY USE SPOT METERING with birds and other wildlife animals, whose head is always in movement. So doing our best to focus on eyes, as they sit with their tails towards u. Here u hv to struggle for correct focusing on their eye(s), beak to get the mouth sharp (not to disgrace the bird or ur picture with focus on tail or wings, the mouth less recognizable being fuzzy).
In the picture above,same thing happened....While keeping vigil on eye or beak, as I clicked, the focus with even middle aperture of f8, maintaining higher shutter speed avoiding shake with weighty lens and camera, the focus on eye missed by few centimeters, as also the bird was moving its wings, which came nicely blurred. I took this picture as an example, with the bird's companion just left the perch and filled the space, which otherwise, wud hv been a negative space and I had to crop the image further to keep the bird at ''RULE OF THIRDS'' The flying bird made my picture stronger, though oof, becoz of off focus from the camera lens.. The wire also is slight diagonal putting the picture in two dimensions of different width and length. Here the eyes comfortable move from short area to broader one and vice-versa.
I sometimes apply vignette in pictures, but avoid in sky picture where space does not need confinement.
(You can kindly view old posts for references,as also the subjects may be repeated with change of time.)