Because they are in Ubers, Ho-oh, Lugia and Yveltal are not allowed despite being birds.
The Lotad family are not included as the only bird-like feature they have is a beak. Lotad is also based on a Lillypad, Lombre is based on a Kappa and Ludicolo is a Mexican Pineapple.
Sigilyph is nothing like a bird.
Archen and Archeops are kind of like Proto-birds. They will be overpowered anyway.
Togetic and Togekiss are closer to flying eggs than birds.
The Porygons are more virus than bird.
Cresselia is more moon than bird.
''
Even without its Mega Stone, that chicken is whack.'' -The Eevee General 2014