As a boy in Somerset in the 1950s I learnt the following.

Whur be thik blackbird be? I know whur e be. E be up thik Wurzel tree and I be ar'er e. E sees I and I sees e, and he knows I be ar'er e. Wi' a bloody girt stick I'll knock e down, blackbird I hav e.

'Thik' is pronounced with a short 'th'

As country folk often netted blackbirds and thrushes for food up to the 1st war (and sometime afterwards I guess), it is possible they were also knocked down. Rook breast meat was also used in pies - I came across somebody cooking rook pie in the English Lake District in the 1970s. Hence the reference to blackbird pie in some versions.