http://revach.net/article.php?id=5028

Section: Parshas HaShavua   Category: Numbers & Letters
Parshas Mitzora: Rav Chaim Kanievsky - The Mitzora's Tzipor, With Or Without A Vav?

A Mitzora brings two birds to become Tahor. One is to Shecht, and the other is kept alive and used to spritz the blood of the dead bird on the Mitzora.  That bird is then let free. By the Shechtia of the first Tzipor (Mitzora 14:5) the Torah spells tzipor with the letter "Vav" in it. When referring to the live one (14:6) the Torah spells tzipor without a Vav. Why?

The Gemara (Yuma 62b) says that both birds must be identical. "Yet," says Rav Chaim Kanievsky, "no two birds can be exactly identical, even two grains of wheat cannot be identical. They are a close as can possibly be. The Rambam says that the nicer one is shechted. That is why the Torah adds the extra Vav," says Rav Chaim.

However, in that same pasuk when dipping the live bird into the blood of the dead bird, the pasuk does not use the extra Vav then. Why? Rav Chaim says,"Because after it is already dead it loses its chashivus."