The flash used during the day will act as a fill in flash and provide a white light against the bird. It will also put more light on the subject and allow for a clearer image. Cameras need light and low light will cause blurry images. Slow shutter speed will also cause blurry images.
Having the aperture around F10 will give you enough focus area but not too much. If you have a higher aperture (F stop/ F number) eg: F20, the camera will have more area on either side of the subject, that will be in focus. This means there is more chance of the camera focusing on the wire as well as the bird.
Slow shutter speed and low light are the main cause of poor images. A shutter speed around 1/100 - 1/250 is typically used for flash photography. For birds and animals, including children that don't sit still, a shutter speed of 1/250 is better. The faster the shutter speed (eg: 1/1000) the less light that will get into the camera. So high shutter speeds like 1/1000 should only be used when there is lots of bright light.
For birds that are just hopping about on a perch or feeding, use 1/250. If you want to catch a bird flying then use 1/500 or higher.
If you are photographing white, yellow or black birds, your camera will hate you
Digital cameras hate subjects that are bright white or yellow, or dark colours like black. If they are against a contrasting colour it isn't as bad but you might find pictures of white finches don't appear that good compared to a normal coloured bird of the same type.Having lots of light on the subject (camera flash, out in full sunlight, etc) can help, with darker subjects. And having shade and using a camera flash can help with lighter subjects.
