Because birds and other flying creatures fly over plants.
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You can counter a grass type with a flying type that would give you the best advantage because grass moves are weak against flying types and there weak against flying type moves.
Serperior is a grass type pokemon, so it is weak to flying type moves.
Grass type Pokémon are weak to Fire, Flying, Ice, Bug and Poison. Ground types are weak to Water, Ice and Grass. So both types are weak to Ice.
Fire, flying, bug, ice, and psychic. I hate grass Pokemon they're so weak!
The Pokemon that are weak to bug Pokemon are Pokemon that are the grass, poison, or psychic type.