You can encounter Sawk on Pokemon White in Shaking/bubbling spots in Route 10, 15, 18 and in Pinwheel Forest.
Also check out this sites Great Pokedex with information about pokemons including locations.
sawks pokdex: http://pokemondb.net/pokedex/sawk
this sites pokedex: http://pokemondb.net/pokedex
shaking grass in route 10, 15, 18, and pinwheel forest
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