Let me assure gnu and Little Hawk that the method described is most humane, the pheasants not selected for the pot all make a full recovery, such would not be the case if fumes from a vehicle were used.
The birds taken by a responsible poacher are the lucky ones, hundreds of thousands are hand reared annually and released to the wild at the start of the pheasant shooting season and then a slaughter commences which can only be compared to that which took place during the 1st WW only in this case one side cannot fight back.
The hanging of the birds for a three week period may or may not take place nowadays but was certainly practiced in "Country Houses" to my personal knowlege in the recent past. I often wondered how the servants could put up with the smell, then again if one takes to loading a manure heap by hand into a cart one does not notice the smell after three or four minutes.