These two swallowtail butterflies are similar and people sometimes confuse them. By good luck, photographer Phil Rowntree and I each snapped one of them separately on the same day. On the left, above, is the Western Tiger Swallowtail (Papilio rutulus) and on the right is the Anise Swallowtail (Papilio zelicaon). They’re in the same family and the same genus and live in the same territory, roughly the northwest quarter of the U.S. The Tiger is a little bigger. One important difference is that the Tiger has a broader range of plants where it lays its eggs for its caterpillars to feed on. The Anise is more picky, strongly favoring plants of the wild carrot family (Apiaceae), especially fennel. In the photos, the tiger is perched on what looks like wild radish, while the anise is on fresh green foeniculum vulgare (fennel). Here are larger pix for butterfly enthusiasts:
Updated June 21: And speaking of butterfly enthusiasts, I happened to see two people peeking intently at a fennel bush on the western ridge. What are you seeing? A butterfly egg, was the response. Sure enough, they showed me a single egg the size of a pinhead poised in the V of a tender newish fennel twig. How in the world did you see it? We saw the butterfly lay it last week, was the response. Here’s my photo of the egg, almost certainly an Anise Swallowtail.