So what's behind knot number one ?

[clint53]

Former residence of a bird.

 

[philoshop]

I usually get squirrels. They can break your focus when you're sawin' while they're trying to leave. Gotta pay attention!!

 

[A.S.Woodchucker]

I was helping a neighbor cut up a storm downed maple branch and ran my saw right through a bird nest with a few birds in it...it isn't pretty! I couldn't figure out what was all over my pants till I saw the other birds scattering!

 

[deutzman]

Was felling a lolly pop white oak that I expect was over 200 years old and was drenched with black water from the hollow inside. I'm talking 25 gallon range. Had to stop, get a bath and put on clean cloths. It didn't run out it sprayed out fast.

 

[woodchuck357]

Most likely a hollow limb stub left for a while, then trimmed back or fell off to allow the callus to form over the hole.