it is a quick washing of the genitals just to make them fresh in the chance you may get action. bird bath b/c it is a small bath and b/c if you wanted to you could dip your balls in a bird bath (or the sink) and wash them. The intent is to wash away potential ball sweat which can dissuade a lady from give a bj

Werd, I will meet you at the bars, I just got to take a quick bird bath.

Achieved by taking your lady friend from behind, either on her knees or stomach. One pulls out prior to busting, aiming their shaft in the direction of the small of her back and proceeds to unload. The small of her back will cup the load allowing a pooling effect which is known as a birdbath.

“three guys later, her birdbath overflowed and ran down the side of her leg.”