All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
In principle, it is not permissible to kill animals, birds or insects that do not usually cause any harm, unless they cause an unusual harm.
It does not appear [from your question] that this bird has done something that makes it permissible to be killed. So whoever killed it should repent to Allaah.
Al-'Izz Ibn 'Abdus-Salaam was asked: "Is it permissible to kill a harmful cat? He replied: "If its harm is beyond the usual harm of cats, and its harm is repeated, then it becomes permissible to kill it."
Allaah Knows best.