Thanks for the post. After seeing the comparisons, I think that I would like to be above 300mm but I may be able to live with my 250mm. I should be able to get closer to the animals in the Galapagos, than elsewhere. I just have to remember not to infringe on their space more than I already am.
Birds in flight may be a different issue, but regardless of the lens I don't think that my current skill would get me sharp images (except by accident). I will stick to mainly grounded birds.