I recently received an inquiry about butterflies in the hills, and want to summarize what we know.
First off, the hills appear to represent an important reserve for native butterfly populations in the L.A. Basin, with 40 species confirmed and a handful of potentially-occurring ones. Diversity of species, not surprisingly, is highest where there is an intersection of major habitat types together, such as willow thickets next to oak woodland next to cactus scrub. Numbers of individuals are dependent on the availability of flowering plants, and some specialized species fly only where their foodplant is in flower.
Not much is known about the distance butterflies can move. A few engage in long-distance migrations, most famously the monarch, but others probably spend most of their lives around the same small patch of native scrub, generation after generation. These specialists are of great concern to conservationists, since their presence often indicates a particularly robust ecosystem.
Many species in the L.A. area fly year-round, but if there's a season when the highest numbers of species and individuals are flying, it's probably late spring (April through June), corresponding to wildflower blooming and warming nights. However, several species emerge through autumn into early winter, nectaring on fall-blooming buckwheat.
In our area, places that attract butterflies include patches of coast buckwheat and mulefat shrubs (only when in bloom), and damp spots along canyon bottoms, particularly if they're exposed to sun. Butterflies will often come to muddy spots in roads to ingest minerals from the soil (most dramatically in the tropics, but also locally).
Most popular gardening books encourage people to plant butterfly-attracting plants to bring butterflies to their gardens, especially "butterfly plant" (a generic term for several species). Several of these are noxious pests (e.g., lantana), which should not be planted anywhere near the open space of the hills. It is better to use native plants (watch for sales at places like Rancho Santa Ana Botanical Gardens in Claremont, or at the L.A. Co. Arboretum in Arcadia), or if your neighbors will stand it, let a portion of your backyard turn "wild" and see what comes in. Just tell them the butterflies made you do it.
A couple websites: