There are ten birds in the tree. How many birds are left when one is shot and killed?

One day, when the teacher tried to test the students' IQ in class, he asked a boy, "there are ten birds in the tree. How many birds are left when I shot and killed one ?"
The boy asked, "Is it a pistol without sound, or is it a gun without sound ?"
"No ."
"How big is gunshots ?"
"80 ~ 100 dB ."
"That means it will shake your ears ?"
"Yes ."
"Isn't it illegal to beat birds in this city ?"
'No offense ."
"Are you sure the bird is really killed ?"
"OK." The teacher is impatient. "Please, just tell me how many are left. OK ?"
"OK. Are there any deaf birds in the bird ?"
"No ."
"Is there a problem with the intelligence of a bird? I am so stupid that I don't know how to fly when I hear a gun ?"
"No, I have an IQ of more than 200 !"
"Are they in cages ?"
"No ."
"Are there any other trees on the edge, and are there any other birds on the tree ?"
"No." "ten miles away ?" "That's a tree !"
"Are there any birds with disabilities or ELE. me flying ?"
"No, it's amazing ."
"Isn't it a bird in a pregnant stomach ?"
"All are public ."
"Is it impossible to get pregnant ?"
"......, Never possible ."
"Are there any flowers in the eyes of a bird? Ten are guaranteed ?"
"There are no flowers, just ten." The sweat on the teacher's head has flowed down,
The bell rings, but the boy continues to ask, "Are you stupid and not afraid of death ?"
"They are all afraid of death ."
"Was it because a couple was hit and they left behind ?"
"Stupid, I didn't mean they were all public !"
"Can comrades !"
"……, Sexual orientation is normal !"
"Will it take one shot to kill two of them ?"
"No ."
"How about killing three at a gun ?"
"No ."
"What about four ?"
"No !"
"What about five ?"
"Absolutely not !!!"
"Are there always six possibilities ?"
"Unless you are a pig, it's possible! Only one shot can be killed !"
"... Okay, so can all birds be moved freely ?"
"Yes ."
"Will they be panic and hit each other when they get scared ?"
"No, every bird has a satellite navigation system and can fly automatically ."
"Well, if your answer is not a lie," the students replied with confidence, "If the dead bird hangs on a tree and does not fall down, there will be one left. If it falls down, only one is left."
The teacher pushed his glasses and endured the feeling of fainting. trembling, he said, "You can become a programmer ......"