There are resting places for over-wintering Monarch butterflies all along the coast of California. These Monarchs have migrated here from places west of the Rockies, while Monarchs from east of the Rockies overwinter in Mexico. (Here's a map for the fall migration and another for the spring.) They like to cluster in the tall trees particularly the eucalyptus even though that is not a native to California. They will cluster in pine and cypress trees as well. The Monarchs that overwinter live about 6 months, while later generations of the spring and summer will only last 2 to 6 weeks.
I checked out Monarch preserves on the web and found a few others that would be along my route and one right at Morro Bay in Morro Bay State Park. So my first stop heading north was at the Elwood Butterfly Grove just north of Santa Barbara. It turned out to be a lovely place for a short hike and lunch, but except for a few butterflies flitting about way up there, I didn't find any clusters. I later learned that there is one particular tree that they like at Elwood, and there was no sign pointing out which one it was, and no knowledgeable docent to help me find it. I don't think at that point that I knew what to look for anyway.
I tried again at Morro Bay State Park where I was told the butterflies like to cluster around campgrounds number 136 to 140. There is a lovely grove of eucalyptus and other tall trees at that end of the campground and I did finally find two small clusters of butterflies near the trash cans!
My last chance to find the type of clusters I had envisioned was to stop at Pismo Beach on my way home. At last I was rewarded. Not only were there thousands of butterflies hanging from the trees, but as the sun warmed up the air, they started to open up, flit about, and even mate. The count, however, is down from previous years. As of December 31st, they have only 11,400 when in previous years they have had as many as 100,000 and more. I asked one of the docents how they count them and it turns out it is a fairly accurate number. They capture a whole cluster in a net and go through and count the butterflies in the net one by one (and even tag them!) then multiply the total number of butterflies by how many other clusters they see.
Here is a male butterfly. You can tell by the narrow black veins on his hindwing and the black "scent gland" on the hindwing. It is unknown what this gland is used for in a Monarch.
This is the female. Her veins are broader. Below are two butterflies starting to mate. When he finds a female that he likes, the male will chase her down to the ground where they will toss and turn until he has her firmly attached to himself. You have to be careful where you walk in a Monarch grove!
He then flies up into the trees with the female hanging from him upside down. It is in the trees that the final act actually takes place. The female's job then is to go off and find some milkweed on which to lay her eggs.
Host plants in the milkweed family are essential to the developing larva; without it, they would not survive. However, there are many species of milkweed, and monarchs can eat most of them. They also eat a plant that is not called milkweed, but is in the same family: Cynanchum laeve, or sand vine.
Among the reasons given for the low counts in recent years are the ongoing drought conditions in the west and the loss of milkweed habitat. The Pismo Beach website has a very informative Q&A page that covers most of the basics of the life of these butterflies.