Palamedes Swallowtail - Papilio palamedes
Looks initially like the Black Swallowtail that we see in the garden every year and whose caterpillars eat our fennel, parsley and dill. But its a little browner and now I can see better it has white lines down the abdomen as opposed to the lines of dots on the Black Swallowtail. What I hadn't noticed on previous occassions were the red antennae. This area is the Northernmost edge of the species range.