function declared in a protocol, in a protocol extension, and in a class/subclass

Question

With the following code:

protocol Flier {
    func fly()
}

extension Flier { 
    func fly() { print ("flap")}
}

class Bird: Flier {
    func fly() { print("flap-flap") }
}

class SmallBird: Bird {
}

let bird: Flier = SmallBird()
bird.fly()

The console prints "flap-flap" which is what is expected.

Why does console prints "flap" if I declare the same function in a subclass instead of declaring it in a superclass?

protocol Flier {
    func fly()
}

extension Flier { 
    func fly() { print ("flap")}
}

class Bird: Flier {}

class SmallBird: Bird {
    func fly() { print ("flap-flap")}
}

let bird: Flier = SmallBird()
bird.fly() //flap