... How would anyone know that?
... The joke is a play on words [Cambridge Dictionary] on various definitions of murder.
A group of crows is called a murder. [Wikipedia]
Two is not quite a group, hence an attempted murder.
To further beat the joke to death, murder also means homicide, and attempted murder is a crime in British and United States' penal codes. The unusual combination of birds and crime adds to the humor as a non-sequitur. ...