It's not just bird people...
Shrewdness of apes
Troop of baboons
Shoal of bass
Cauldron of bats
Sleuth of bears
Sounder of boar
Gang of Buffalo
Caravan of camels
Destruction of cats
Coalition of cheetahs
Bed of clams
Quiver of cobras
Rag of colts
Float of crocodiles
Pod of Dolphins
Parade of elephants
Mob of emus
As for why? Blame authors (mostly poets) for wanting to use more colorful language in describing different groups of animals over the past several hundred years.
Think about it, in writing something, poem or book, does it sound more interesting if you use herd or group of goats rather than writing a tribe of goats?
The writer/ author will more often lean towards the more creative and colorful side. Some of these terms get accepted into everyday language use... some don't, but they all become accepted names over long enough time even if they don't achieve everyday use.
Pride of Lions is a more recent example though, where accepted use only became official in 1930, and literally because of that, it surged to the primary common use term for Lions, when before it was accepted, group, herd or pack were all more commonly used.
Most, though official, don't reach that level of common use. Everyone is aware of murder of crows, but most still call them a flock instead.