Duck typing problem

class Duck( object ):
    pass

class Farm:
    ducks = []

    def addDuck(self, duck):
        getattr(duck, 'fly')
        getattr(duck, 'quack')
        getattr(duck, 'swim')

        self.ducks.append(duck)
        print "Add a duck"

aRealDuck = Duck()
setattr(aRealDuck, 'fly', 1)
setattr(aRealDuck, 'quack', 1)
setattr(aRealDuck, 'swim', 1)
setattr(aRealDuck, 'feathers', 1)

aFakeDuckWithoutFeathers = Duck()
setattr(aFakeDuckWithoutFeathers, 'fly', 1)
setattr(aFakeDuckWithoutFeathers, 'quack', 1)
setattr(aFakeDuckWithoutFeathers, 'swim', 1)

f = Farm()
f.addDuck(aRealDuck)
f.addDuck(aFakeDuckWithoutFeathers) # No error!!!!

# ... 15.000 lines and 2 years later, ducks go to slaughter ...

class Slaughter:
    feathers = 0

    def cleanDuck(self, duck):
        self.removeFeathers(duck)
        print "Duck dead =("

    def removeFeathers(self, duck):
        getattr(duck, 'feathers')

        self.feathers += duck.feathers
        duck.feathers = 0
        print "Feathers removed"

s = Slaughter()
for c in f.ducks:
    s.cleanDuck(c) # Crash...

#Add a duck
#Add a duck
#Feathers removed
#Duck dead =(
#Traceback (most recent call last):
#   File "/home/nicolas/duck.py", line 48, in <module>
#   s.cleanDuck(c) # Crash...
#   File "/home/nicolas/duck.py", line 36, in cleanDuck
#   self.removeFeathers(duck)
#   File "/home/nicolas/duck.py", line 40, in removeFeathers
#   getattr(duck, 'feathers')
#   AttributeError: 'Duck' object has no attribute 'feathers'