@ Lurker: I think it's more of a baseless assumption that they don't have one than they do.
Tealove is best pony.
Pony Fanon Index
Given what conclusive evidence? Given you're trying to prove something exists that hasn't been shown to thus far, the burder of proof lies with you.
Based on the entirety of the episode Dragon Quest.
Tealove is best pony.
Pony Fanon Index
A migration that is stated to be a rare occurrence and thus an exception to typical dragon behavior is not a conclusive indicator of anything.
A migration that shows that dragons can interact perfectly well from day one of it. A migration that can't possibly be just about breeding because young dragons are there as well. A migration that shows that dragons communally watch over their young.
Tealove is best pony.
Pony Fanon Index
Oh, I could wade in here, but I fear that we'd get bogged down.
Tealove is best pony.
Ask The Mane Six
Bad: But as Teeth pointed out, the sample size is too small. And aside from that, Spike was stealing and eating his stuff. I can't think of many people who would be okay with that and I can see this especially pissing off a species known for hoarding. In fact, the Dragonshy one had a similar reaction when Rarity tried to sneak off with some of his stuff.
@Perpetual: I agree to the point that we can't argue for or against its existence. I do think there are hints that there's a large infrastructure to dragon culture, but how far it extends and whether it qualifies as "civilization" is totally up for interpretation.