2) R’ Chaim Kanievsky in Tama Dekra explains that there is a mitzvah that the two birds have the same in appearance and size (like we find in Yoma 62b). However, it’s impossible to find two identical birds, as even two grains of wheat look different (see Yerushalmi 4:9), however, one has to do his best (Tosfos Yeshonim beginning of Shnei Se’iray).
Being that they aren’t identical, it must be that one is nicer than the other. The bird which looked nicer was the one that was slaughtered (Rambam, Tumas Tzora’as 11:1). Consequently, when the pasuk refers to the bird that was slaughtered it is written with a ‘vav’ as it was the nicer one, however, the one which was sent away wasn’t as nice, therefore, it is written without a ‘vav’.
However, if this is peshat, how come later on in the parsha, the Torah refers to the slaughtered bird as: וטבל בדם הצפר השחוטה – “dip into the blood of the slaughtered bird”, without a ‘vav’?
R’ Chaim explains very simply, that once the nicer bird is slaughtered, it is now forbidden to benefit from and it loses its importance, therefore, it is now referred to as הצפר without a ‘vav’.
