Eagles of the North

Creature — Bird Soldier

Flying

When this creature enters, creatures you control get +1/+0 and gain first strike until end of turn.

Plainscycling ( , Discard this card: Search your library for a Plains card, reveal it, put it into your hand, then shuffle.)

Eagles of the North's triggered ability affects only creatures you control at the time it resolves. Any creatures that come under your control later in the turn won't be affected.
Unlike the normal cycling ability, typecycling doesn't allow you to draw a card. Rather, it lets you search your library for a card with the type or types indicated by the ability name. For example, a card with basic landcycling lets you search for a basic land card, and a card with Wizardcycling lets you search for a Wizard card.
Typecycling is a form of cycling. Any ability that triggers on a card being cycled also triggers on a card being typecycled. Any ability that stops a cycling ability from being activated also stops a typecycling ability from being activated.