A flightless Bird ... or ...

... how to leave book with a trappy move that should have lost the game.

What happens if White tries d4, either immediately or with a small delay? What are plausible plans for each player?

6.Nf3 I just don't see what's wrong with that move.

I like 6.Nf3 too - it safely delays the d2-d4 decision (without giving anything up that I can see) and develops a piece. Black follows with Nc6, and now White must decide: d2-d4, or not?

You've got the wrong animal... play 4.e6 and you've got the hedgehog. Tougher than the Bird by quite a bit IMO.

I don't know if that position you arrive after6.d4 is so unplayable for Black. Black wants to get in c5 I think and break up that White center before White can play e3. I don't think the immediate 6....c5 works because of 7.Ba3!? ...interesting position.

The cliche with White is is you've got a pawn on e5 your plan is to attack the kingside. For White I'd be thinking 0-0-0 and send in the kingside pawns. For Black I'd play Be7 and pray to God that he kicks the N with h3(??) -- or maybe even play h6 and immediately grab my head and say "omg my N doeesn't have anywhere to go... god, why'd I tell you!?" ;)

what's wrong with 6.e3?

although, i admit Nf3 looks just as good.

and my other curiosity is...what is the trap (if there even is one) o.O

White would like play d4 and defend d4 with e3... but he can't play e3 because of the pesky N on g4... it might be tempting to kick the N on g4 with h3 (if you're <1500)... but h3 leads to Qh4+ with mate to follow.

Nf3 sure looks like commonsense. I wonder if the immediate d4 isn't better, though, because of

6.Nf3 c5 (I think black has to challenge that center) now 7.Ba3 has no teeth because 7...d4

Whereas:

6.d4 c5?! 7.Ba3 and I think Black has issues.

I should mention that I'm not using an engine and I make LOTS of errors when I analyze, so have at it. I'm perfectly willing to believe my whole take on the position is flawed.