When metering the shot consider that the bird, if you're shooting upwards toward the bird and the sky, is likely going to be in shadow compared to the sky around it. Thus the meter (in most modes) is going to underexpose the bird; so you've got to tell the camera to overexpose a little (either manually yourself or through exposure compensation). Checking the histogram is important to avoid blowing the sky out totally; but you can get more underside detail to work with if you overexpose.
Of course you've got to watch the bird and light angles; depending on your position the bird could be illuminated as well if the sun is low so do keep that in mind; but in general the sky will nearly always be brighter than the bird.
Otherwise its a case of a long lens, good focusing and steady shooting. A tripod can help as it takes the weight off you and if you're using a heavier setup also a lot of the strain.