I was very pleased to see him. Swallowtails are one of those things that we should preserve for purely aesthetic reasons, damnit--it's a bird-sized piece of origami in black and bright yellow. They lack the personality of our militant little morning cloak, but for sheer glory, they're hard to beat.
If my reading is correct, he would have spent the winter as a chrysalis, probably under the leaf litter, and only just pupated now that the weather's warm. (This is another of the prices we pay for our lawns--we kill a great many insects, including butterflies, by destroying leaves. Fall is a regular chrysalis apocalypse in the suburbs. If you can rake your leaves somewhere--under the trees, or whatever--and leave them, rather than bagging them or burning them or whatever, that helps bugs. And also makes much better dirt.)
I'm feelin' kinda blah today--throat's still sore--so once I've finished off my obligatory Batbreath and done a turn around the garden, I'm gonna go lay down for awhile.