Hi Sue, I was going to suggest the 100-400 f/4.5 which would be ideal for your bird photos. I always considered this to be a lightweight lens, That was until I checked out the specs, Twice the weight of the 70-300mm at 1380 grams.
So how about approaching your problem from another angle and look at the Op-tec Pro camera straps.
http://www.optech-online.co.uk/lst67.htm The blurb says "Equipment instantly feels 50% lighter and 100% more comfortable when using a PRO STRAP™" .
I use these straps on all my cameras and agree with the above statement.