To add to this... animals think via patterns. I don't know anyone that actively disagrees with this theory, which is why I feel comfortable stating it. Their reactions to humans/objects/places are determined by past experiences, either with the exact thing or with similar things. They are VERY good at analyzing patterns as relate to them. They are not quite as good at independent patterns (but they still can figure them out somewhat.) Approach/leave basically rubs in that pattern of familiarity and safety in a much more rapid manner than a once-a-day dump-feed-and-leave interaction will. By not directly interacting with them (in my case, I work on a construction project outside while ignoring the birds) their curiosity is also piqued. They want to know what you're up to. This is an important state in animals. Highly reactive animals are also highly curious when their flight reaction isn't tripped. When any animal is purely curious, they are not gearing up for aggressive behaviour. The formation of patterns is extremely malleable when they are in this state, in my experience. I believe this is why my Leghorns are almost annoyingly friendly when so many others write them off as crazy. I think quite a few of my birds haven't been picked up in months, yet will still come streaking over when I approach. Some even sit on my lap. I rarely use treats to encourage them to be around me; I find it encourages affrontive behaviour in both hens and roosters if they come to expect it and it's not there. Their food goes in and stays in their bowls.
A small detour, if you'll tolerate it: with ducks (perhaps not drakes—I don't keep many, so I wouldn't know) there is no risk of aggressive behaviour like there is with roosters, so I actively exhibit deferential behaviours towards them when interaction has to be direct. If they're sitting down and I need to walk by, I slow down so that I'm nearly stopped, readjust my course to be at least 2--3' away from them, and walk past at a resumed normal pace with my gaze soft and directed at the ground. Arm movement is kept to a minimum and fingers are in a soft position. I find that the near-stop done before I reach them is a particular help to keeping them calm and still sitting. It's what a lower-ranking bird does when near a higher-ranking bird. If it's a total stop, they'll get suspicious and leave. Ducks are much more sensitive to frozen body language than chickens, who are actually comforted by it to some extent if it's not intense. My ducks are the friendliest when I am kneeling to work on something with my back turned to them. They'll walk right up and climb all over my legs.