Birds in the cannot be borrowed from other players per se, however, certain markers after being scouted may display a mission where you may have an option to borrow a bird from a different player. Such a bird can be borrowed only for this single specific level.
Borrowing a bird like that from another player also grants friends points, which can be exchanged in the ||hatchery|| for additional hatches. Please keep in mind that each hatch is equal to a ||bronze ticket||.