Birds tend to be flying in the sky, and they appear as a dark object with a bright background. If the bird is small, then metering gets very tricky.
The trick is to determine whether the bird is lighter or darker than the background, and then by how much.
The background can be blue sky, white clouds, dark storm clouds etc, while the bird can be from mostly white to mostly dark. As Bob suggests, it's common to have bright skies and dark birds ; this requires + EC, sometimes lots. However, many birds, white or mixed require -EC if the skies are dark and the metering wants to open up the lens. Often there's an enormous peak in the histogram (for a luminance histogram) that is the sky, and it's hard to see the little blips that are from the target; but it's important to look for it and move it as far to the right as possible WITHOUT blowing the highlights. Check carefully for blinkies in the image of the target - it's OK to have a bright sky create a large peak topwards the right - or in the case of a dark bird - some of which goes off the right of the histogram (and leads to blinkies in a lot of the picture). I prefer to get some blown-out sky and a good image of the bird rather than a non-blown-out sky and a dark image which shows lots of noise when tweaked later. As noted, particularly problematic are mostly white birds, such as egrets, against a dark sky or even against the marsh etc, and I've had to resort to up to (sometimes over) -2 EC to keep the bird from blinking and the marsh looks like a night shot. Since they are often only a small part of the scene (unless you have a supertele and are stealthy (or at a zoo!) it's the difference between the background and bird that should determine the approach. If the bird "matches" the background, the metering's likely to be close...
(I thought Art Morris's website had some guides for how much EC under general conditions, but couldn't find them any more)