simple solution, dont fly a glider ive had birds of prey look at my ENGINE powered planes while i was flying (even follow me around) but once they get close enough to attack they realize theirs a big spinning knife on the front and usually leave, w/o the big spinning knife you have problems (as you have expierenced).
one time the bird wouldnt leave me alone, i had a spad so i wasnt worried about looseing to the bird, so i went from prey to preditor and started chaseing it. it flew away rather quick .
bird vs engine, i bet on the engine every time