最后一句为什么用"having"?

最后一句为什么用"having"?
“Tell me the weight of a snowflake,”a titmouse asked a wild dove.
"Nothing more than nothing,"was the answer.
"In that case I must tell you a marvelous story," the titmouse said.
"I sat on the branch of a fir tree,close to its trunk,when it began to snow-not heavily,not in a raging blizzard---no,just like a dream,without a sound and without any violence.Since I did not have anything better to do,I counted the snowflakes settling on the twigs and needles of my branch.Their number was exactly 8865220.When 8865221st dropped onto the branch,nothing more than nothing,as you say ---the branch broke off."
Having said that,the titmouse flew away.