Question

a flock of crows is known as muder how?^_^

Answer 1

A group of crows is called a “murder.” There are several different explanations for the origin of this term, mostly based on old folk tales and superstitions.

Answer 2

In the Late Middle Ages (1486), was published The Book of Saint Albans (or Boke of Seynt Albans), a compilation of matters relating to the interests of the time of a gentleman. It is also known by titles that are more accurate, such as "The Book of Hawking, Hunting, and Blasing of Arms".

The book contains, appended, a large list of special collective nouns for animals, "company terms" also known as the terms of venery (venery being an archaic word for hunting), such as "gaggle of geese" and the like. Amongst these are numerous humorous collective nouns for different professions, such as a "diligence of messengers", a "melody of harpers", a "blast of hunters", "a subtlety of sergeants", "a gaggle of women", and a "superfluity of nuns". The tradition of a large number of such collective nouns which has survived into modern Standard English ultimately goes back to this book : a crash of rhinos, a mischief of mice, and a puddling of ducks and of course a murder of crows.

There are several different explanations for the origin of this term, mostly based on old folk tales and superstitions :

For instance, there is a folktale that crows will gather and decide the capital fate of another crow.
Many view the appearance of crows as an omen of death because ravens and crows are scavengers and are generally associated with dead bodies, battlefields, and cemeteries, and they’re thought to circle in large numbers above sites where animals or people are expected to soon die.
But the term “murder of crows” mostly reflects a time when groupings of many animals had colorful and poetic names.
If you have interest in this subject, James Lipton's An Exaltation of Larks is a great reference read, or you can look at this exhaustive page of collective nouns for animals.