Does abnormal sperm shape cause birth defects?

Unlikely. Since millions of sperm are involved in the race for the egg, it is the abnormal sperm that don't swim as well and are not likely to succeed in fertilizing an egg.
Abn sperm/defects. Abnormal sperm shape suggests "defective" sperm/abnormal sperm content which could result in infertility or possibly "defective" conception: you would never see an infertility center inserting an abnormally shaped sperm into an ovum (icsi/suzi). High "abnormal" counts would generally lead to an "assisted" fertilization.