I shot the Northern Shrike below
against a heavily overcast and misty sky. I metered off the tree trunk,
locked the exposure and then shot. .........
I kept going back and
metering off the tree trunk, but most of the photos were way
Since I am not exposed to the field conditions as you were (while taking those pics), it is difficult to say what went wrong! However, based on what you said, here are some thoughts!
1. By metering off of tree trunk, your idea is that it is giving you Zone 5 (Average weathered wood - 18% reflectance) reading. I am not sure that this is always the case (for example, I always had better results by under exposing Redwoods a tad!)
2. Even if your reference is correct, to get the correct exposure on bird based on your reference, the bird should be in the same light as the tree trunk.
3. The fact that the bird is over exposed, I am guessing that the tree trunk is in a shadow effectively making it a Zone 4/3 object. Your camera (erroneously) compensated for the dark object by giving you a reading 1-2 stops over! Bracketing would have helped! However, as RLFarhy pointed out, with Digital, a reference shot would have greatly helped (no need to bracket!).
4. Personally, I would try this - assuming your white balance is set to Daylight. Put the camera in manual metering mode as said by the previous poster, start with Sunny 16 rule, open 2.5 stops for the kind of overcast sky you described, note down that exposure and then play with Shutter speed and Aperture for that reading! Take a shot and adjust exposure as (if) required.
Hope this helps!