https://www.chess.com/forum/search?keyword=bird+opening
is the bird (f4) a sound opening?
"... the Bird ... has never had a great following at any level. Nevertheless, for players who know what they are doing, it offers White excellent attacking chances and good opportunities to shake things up, while forcing both players to think for themselves right from the beginning of the game. ..." - FM Carsten Hansen (2010)

It's just fine, and certainly not theory-heavy.
It's useful to know the ideas of the Dutch defence, to apply them with reversed colors (the Leningrad Dutch in particular).

Black plays a KID with ...e5 and has the easier game.

As long as you know the ideas of the opening, and have played it with success it should be quite a fine opening. I play the dutch, and many gambits with nice success, so I think the bird should be completely playable. Good luck!

"... the Bird ... has never had a great following at any level. Nevertheless, for players who know what they are doing, it offers White excellent attacking chances and good opportunities to shake things up, while forcing both players to think for themselves right from the beginning of the game. ..." - FM Carsten Hansen (2010)
Heh, a perfect quote for the Bird from an FM. You won't get this quote from a super GM ...
I love the Bird and have opened with f4 much of my life, but it's not viable at top levels. It gives away white's quarter pawn advantage from move 1 on. It's a great opening at master and below, but at high levels it's like handing the opponent equality and then fighting to prove you can get it back the whole game.

Bird is perfect for blitz, maybe for rapid too.
There is very limited number of variations, and once You get to know how to defend against From's gambit you're secured
Long time controls is another story, since 1.f4 starts with ~ -0.20 for white, opposed to 1.e4 or 1.d4 which are both + 0.20.
In any case Bird game can't be boring, because each move is deciding, especially in From gambit variations.

"... the Bird ... has never had a great following at any level. Nevertheless, for players who know what they are doing, it offers White excellent attacking chances and good opportunities to shake things up, while forcing both players to think for themselves right from the beginning of the game. ..." - FM Carsten Hansen (2010)
Heh, a perfect quote for the Bird from an FM. You won't get this quote from a super GM ...
I love the Bird and have opened with f4 much of my life, but it's not viable at top levels. It gives away white's quarter pawn advantage from move 1 on. It's a great opening at master and below, but at high levels it's like handing the opponent equality and then fighting to prove you can get it back the whole game.

Do you guys the Leningrad bird variation or classical Dutch variation is better and why?

If Carlsen can play the dutch, you can play the bird.
Some 'openings' don't lend themselves very well to the 'extra move'. This I believe is one.

And it's Black's choice to have a Dutch reversed or not.
But there is a lot of room for original play in the Bird, I'll give it that.

<<Long time controls is another story, since 1.f4 starts with ~ -0.20 for white, opposed to 1.e4 or 1.d4 which are both + 0.20.>>
That's impossible and just shows how chess engine position evaluation is flawed.
Chess engines do now what is already done by great masters before.
There is a reason why GM-s stick to 1.d4 and 1.e4 after so many centuries. I presume it's because of slight advantage with these two, over equal opponent.
Of course that Bird is not "unsound" but rather with less options.
If not, why specifically is the Dutch defense (f5) considered sound by some top level players?