Main idea of Bird's opening I want to ask a question about Bird's opening (1. f4). My question is:


What's the main idea of this opening?
Is 1. ... d5 the best counter to 1. f4? Why 1. ...d5?
What's the main idea of this opening? 


Main ideas for white:


surprise weapon as it is rarely played (since it is not all that good)
playing the Dutch opening with an extra tempo, but this is not a big deal IMO
avoiding opening theory as you can basically learn a "system" (certain position of pieces that you want to achieve) and follow this almost independent of what black plays
in some cases the pawn being already on f4 can be handy for kingside attacks


Basically there are three set ups:


stonewall with pawns on f4, e3, d4, c3; gives you good control of dark squares but weakens the light squares; Usually you only play this if black plays d5 at some point as otherwise black has the option to play something like d6 and e5 which is often unpleasant for the stonewall setup
fianchettoed bishop on  b2 often in combinations with pawns on e3 and f4. This gives you good control along the a1-h8 diagonal 
fianchettoed bishop on g2. This can lead to king's indian like setups if you combine it with d3, e4.


In the end it depends whether you like the resulting positions. Check a few games from the database and look at the positions around move 10.


  Is the best counter of 1.f4 is 1...d5? Why 1...d5?


Yes, d5 is most popular and best. It takes control of the center, opens the way for the light squared bishop and is just a natural move. A reason not to play 1. ...d5 could be if you are worried about losing control of the dark squares in the center. In that case you could think to wait with d5, perhaps develop the bishop to g7 (which would work well against the white setup number "2" above) and leaving the option open to play d6 e5 later which would work well against the stonewall.

As a former 1. f4 player I would discourage you from playing this or any other off-beat opening if you want to be serious about chess. You can learn a lot more (and get better positions) if you play more normal openings.