If it were one thing, I'd guess it's the AnimNodeSequence object in the DefaultProperties. UTPawn already has two AnimNodeSequences called MeshSequenceA and MeshSequenceB, so trying to overwrite MeshSequenceA might be causing the problem. Try commenting out those two lines in your pawn's DefaultProperties (right above the commented-out DynamicLightEnvironment) and see if that helps.
Failing that, there's always the obligatory "make sure the animation actually works in the editor" advice.
Also, I've heard that DefaultProperties doesn't like spaces with declarations, like "BirdWalkAnim = WalkOnLand"... Apparently it's better to write "BirdWalkAnim=WalkOnLand". I've personally never had any problem with it either way, but can't hurt to try, even though it's probably not the source of the problem.
Hope something here works for you