Many people are tempted by 10 or 12 power binoculars, these are great, but unless you have a lot of experience with binoculars, these will cause you great difficulty locating the bird since they focus in on a smaller area. They also increase the effect of hand shake. The larger the lens, the greater the amount of light entering the binoculars, thus the better the image. Neck and back strain would surely detract from your enjoyment of the birds. The barrels should be easily flexible, but not so flexible that they fall open when holding onto one side.
Coated lenses reduce internal glare and increase the amount of light.
Hold the binoculars a foot away and look for a bluish or purple tinge to the lens. You should be able to bring the barrels close enough together so that the images merge into one, single, clear image within a perfect circle, Improper binoculars can cause eye strain and headaches. If you wear glasses, make sure the rubber cups around the eyepieces fold back to accommodate eyeglasses. Get binoculars that produce a clear image of an object as close as 20 feet,or you may miss some birds. Look for minimal image distortion at the edges of the lens.