Monarch butterflies are famous for their long migrations. These butterflies cannot survive cold weather, hence they travel long distances. Some monarchs survive from 6-8 months & can fly as far as 3000 miles! From early october these can be spotted at natural bridges state beach in Santa Cruz
Tele lens are highly recommended if you are planning to photograph these. I used nikkor 70-300 lens & also thanks to my friend @Shivanand Kini for letting me use his Tamron 150-600
Check out some sample images!