Just a theory :

When you feed your chocobo you can get a "growing feathers" message. Once you got it you *always* get it in the same session. I think it's just to notify we fed our chocobo enough for him to pass a step, and change color, and that's why it's always displayed after the first one.

In my opinion, all feeds are effective, but only a certain amount will allow to pass to another color.
It also means that the 2nd message and the other ones are just a repetition of the first, so furthest messages are irrelevant to the effectiveness or not of the last given fruits !

I don't think it's relevant to note "growing feathers" messages like that. Either people STOP feeding the chocobo and wait to see each color each time it says "growing feathers" to really note when fruit counted as a big step, either it's no use.

PS: some people have noted the message went away after feeding a lot, I think that's just because they went back to current chocobo color with all their feedings ^^