In the Bird's Defense Deferred in the Ruy Lopez(1.e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nd4), why can't White take the e5-pawn?
In the case of 5.Nxe5 Qe7 6.f4 b5, Black will meet both 7.c3 and 7.Bb3? with 7...d6. E.g., 7.c3 d6 8.cxd4 dxe5 is apparently around equal.
A better response for White is 5.Nxd4 exd4 6.0-0. This is an improved version (for White) of the Bird's Defense, since now ...c6 won't come with tempo. In the normal Bird's Defense with the bishop on b5, ...c6 was a key resource.