-------------------------------------------------------------------------------------
Birdshot = a shotshell with small pellets which are used to mostly hunt moving (flying/ running) small game - rabbits/ hares, birds etc. Since a large % of the time this is what is used for wing shooting ducks, grouse, turkey and all manner of birds it is called birdshot.
Birdshot is also what is used in competitive shotgun sports like trap & skeet.
Buckshot = a shotshell with much larger pellets (compared to birdshot) - which is used to hunt larger animals. Used to hunt deer as well... hence the name buckshot.
The numbering of shot is not a uniform thing, with different countries using a different number for the same shot size. See http://www.sellier-bellot.cz/shotgun-table-shots.php for a table of shot sizes in inches and mm.
In India we follow the British numbering (no surprises there) and (from largest diameter to smallest) for birdshot is BB (called No. 1 in USA) to No.9. For most wing shooting No. 6 is acknowledged to be the most flexible birdshot number.
The most common reference to Buckshot in India is to the two largest shot sizes LG (called 000 buck in USA) and SG (called 00 buck in USA).
For home defence buckshot is what is usually recommended... humans being large animals
LOL -------------------------------------------------------------------------------------