Scala traits as D interface... maybe.

gistfile1.d

import std.stdio;

interface Flying {
    @property string flyingMessage();
    final void fly() {
        writeln(flyingMessage);
    }
}

interface Swimming {
    final void swim() {
        writeln("I swim");
    }
}

interface Bird {}

class Penguin : Swimming, Bird {}

class Pigeon : Swimming, Flying, Bird {
    override @property string flyingMessage() {
        return "I'm a flyer";
    }
}

class Hawk : Flying, Swimming, Bird {
    override @property string flyingMessage() {
        return "I'm an excellent flyer";
    }
}

class FrigateBird : Flying, Bird {
    override @property string flyingMessage() {
        return "I'm a good flyer";
    }
}

void main() {
    auto flyingBird = [cast(Flying)
        new Pigeon,
        new Hawk,
        new FrigateBird];

    foreach(b; flyingBird)
        b.fly();

    auto swimmingBird = [cast(Swimming)
        new Pigeon,
        new Hawk,
        new Penguin];

    foreach(b; swimmingBird)
        b.swim();
}