On the problem of birds taking off in fright

One of the most sensitive ones takes off first, and the others respond immediately.

Supplement:

There is no information, but there is evidence. Like people, all animals are different in intelligence or sensitivity. Since there are differences, there must be one that is the most sensitive. On the other hand, if the birds are scattered, it may be the one closest to passers-by, which may not be the most sensitive.