Why Dust or Oil?

If i’ve posted this in the wrong section please feel free to relocate it, cheers.Ok so this ones had me racking my brains for ages. From a biological / evolutionary perspective, can anyone explain why some birds are absent oil glands an the purpose dust down feathers has as an alternative. I know some birds are a bit more dusty than others, and all birds have down feathers obviously) but im expressly talking about birds that have oil glands (or the Uropygial / preen gland) and those that dont, and the various advantages / disadvantages to each as such.I had assumed it was something the do with the environmental climate, or amount of contact with water etc, but there doesn’t seem to be much consistency with it. I can only speak from my experience with raptors, but it seems that even species of birds that occupy the same territory (an thus environment / climate) there seems to be quite a variations as to which of those have preen glands and which don’t.One thing that i have sort of noticed…ish…is that species that lack a preen gland appear to be those which dont appreciate being touch on the back quite as readily. This would make sense given that instinctive lacking of any tactile feedback on the back to become desensitized to as a result of preening (raking the beak across the back) vs those with an preen gland which become used to the experience.This of course is pure speculation, and the ramblings of someone who may have had one too many mugs of coffee! But anyway, if anyone has any knowledge on this i’d be curious to know the science behind it all!CheersChris

Must admit I always thought it was climate related, rainy enviornments -oil and dry ones - dust.Saying that evolution takes a hell of a long time so maybe they just havent evolved enough yet lol.Seriously though someone else with more knowledge will hopefully come along.