As we met beneath the trees on the hilltop, shadows of these butterflies passed continuously. .... described their "hilltopping" behavior: they tend toward hilltops, where others of their kind congregate. He (name?) said the flitting shadows would make a good screen saver. (I am not completely sure these are not western tiger swallowtails (Papilio rutulus). But these are white, not yellow, and I read that the westerns do not hilltop.)