Migratory Route

Create four 1/1 white Bird creature tokens with flying.
Basic landcycling {2} ({2}, Discard this card: Search your library for a basic land card, reveal it, put it into your hand, then shuffle.)

Glacial Fortress

Glacial Fortress enters the battlefield tapped unless you control a Plains or an Island.
{T}: Add {W} or {U}.

Jötun Owl Keeper

Cumulative upkeep {W} or {U} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
When Jötun Owl Keeper dies, create a 1/1 white Bird creature token with flying for each age counter on it.

Flooded Strand

{T}, Pay 1 life, Sacrifice Flooded Strand: Search your library for a Plains or Island card, put it onto the battlefield, then shuffle.

Arctic Aven

Flying
Arctic Aven gets +1/+1 as long as you control a Plains.
{W}: Arctic Aven gains lifelink until end of turn. (Damage dealt by this creature also causes you to gain that much life.)

Tundra

Emeria Angel

Flying
Landfall — Whenever a land enters the battlefield under your control, you may create a 1/1 white Bird creature token with flying.

Plains

Hallowed Fountain

({T}: Add {W} or {U}.)
As Hallowed Fountain enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

Judge's Familiar

Flying
Sacrifice Judge's Familiar: Counter target instant or sorcery spell unless its controller pays {1}.

Empyrean Eagle

Flying
Other creatures you control with flying get +1/+1.

Airborne Aid

Draw a card for each Bird on the battlefield.

Watcher of the Spheres

Flying
Creature spells with flying you cast cost {1} less to cast.
Whenever another creature with flying enters the battlefield under your control, Watcher of the Spheres gets +1/+1 until end of turn.

Lieutenant Kirtar

Flying
{1}{W}, Sacrifice Lieutenant Kirtar: Exile target attacking creature.

Nimble Obstructionist

Flash
Flying
Cycling {2}{U} ({2}{U}, Discard this card: Draw a card.)
When you cycle Nimble Obstructionist, counter target activated or triggered ability you don't control.

Lofty Denial

Counter target spell unless its controller pays {1}. If you control a creature with flying, counter that spell unless its controller pays {4} instead.

Winged Words

This spell costs {1} less to cast if you control a creature with flying.
Draw two cards.

Pride of the Clouds

Flying
Pride of the Clouds gets +1/+1 for each other creature with flying on the battlefield.
Forecast — {2}{W}{U}, Reveal Pride of the Clouds from your hand: Create a 1/1 white and blue Bird creature token with flying. (Activate only during your upkeep and only once each turn.)