Bird Problem

I suspect its a mating call. The male woodpecker is showing off his work ethic to any female in listening range, hence the pipe medium as water is an excellent conductor of sound.

skeeter

I think they tap until they hear a hollow sound. Once they do, they know bugs made a little nest in there.

Thats Right!!!
A bird would never bang her head against a healthy tree because nothing good will come ot of it. Birds look for hollow and sick trees to search for bugs and a pipe sounds like one.

Good Job!