It probably should be a seperate topic.
First - a quick Latin lesson. The singular of species is species. 'Specie' is not a word used in Biology, but is found in coinage. It's a minor thing I know, but (a) it makes you sound like you don't know what you're talking about and (b) it irritates me no end.
Moving on - the suggestion, as I understand it, is that feathers are the first stage. Feathers do not just enable flight - birds are covered with them, not just on their wings. They
insulate. And this may have been their primary function.
The fact that fossil birds (e.g. Archaeopteryx) with fully functional wings still have functional claws on those wings gives a clue to the further evolution. The forelimbs were just that, albeit feathered (for insulation). In the same way as flying squirrels have done, it is not hard to envisage an arboreal reptile developing a gliding ability. From there, any modification - such as large 'flight' feathers such as those on a birds wing - that assist in gliding, in this case by massively increasing the area of the flight surface whilst adding little to weight, is going to be a benefit.
Since we are talking about gliding, these feathers do not need to be the highly specialised flight feathers of flying birds. But again, these, dependent on the barb and hook mechanism, are not hard to explain. If you are extending a gliding surface by putting feathers on the trailing edge, anything that makes the 'veins' of your feather stick together a bit is going to be of benefit, because it improves the air resistance.
This is conjecture; it may not have happened this way. But it demonstrates there is at least one way it can happen.
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