Who eats a swift's nest? Many many people, in Asia mostly. I don't know, though, if this is what 'bird's nest soup' is all about. The Viet Nam News explains that “Swift nests are high in protein, low in fat, contain various amino acids essential to the human body and other beneficial substances”. You can read about it here:
I don't know what species of swifts are big business in Viet Nam but here in BC, we might see black swifts nesting if we happen to be in the vicinity of a cliff near a waterfall. Nesting is the only thing they do on land; everything else happens in the air, including feeding and mating.
We're more likely, though, to see the Vaux's swift, a small cigar-shaped swift that arrives in BC in the spring and leaves in September. Although believed to breed in BC, only a few nests have ever been found, usually in human-made structures rather than tree cavities, their natural nesting site.
If you have a photo of a Vaux's swift, I'd love to post it here ...