Specifically black bellied whistling ducks. It started when I was 10 or so and my aunt and uncle would take me to the zoo in their town and we would go to the aviary with a handful of 25 cent feed that we got out of a little stand. I would throw some in and the other birds would just eat it, while this little duck would yell at me. I'd yell back at him and tell him the food was behind him and he would just whistle loudly. Then I would whistle at him and then eventually he would waddle to the back.
6 or so years later, they opened up the aviary where you could walk into it. The duck had posters up saying "Warning!! This duck is very territorial!!" ..sadly he then got moved to the swan pond, though I visited him through the fence because he would still be whistling at everyone who passed.
Now I'm 20 and he's back in the aviary with 2 more ducks. But (I'm quite sure it's the same duck) he's still on his own and the 2 other ones are a couple. An old lady sat on a bench in there and he snuck up under her and she said "Which bird is making those little whistling noises?" Then he bit the lady's pants. He didn't bite her, but I happily distracted him by whistling at him and he ran over and loudly whistled at me. The party with the lady said "Oh my...That's loud" So I fed him my millet stick intended for the parakeets, since they were in a place that I had to reach up to get them to eat.
Here he is biting me because I tried to touch him, he starts to sound like a rubber duckie.