Only twice have I been concerned about birds when higher than about 500' AGL. A couple of weekends ago I was climbing through 2500' just south-east of Bagby in North Yorkshire when I saw a large duck or goose a couple of hundred feet above me in the 11 o'clock, heading amost parallel but moving slowly left-to-right. I had clear space ahead and below so dived under him until he was well behind.